[FIX] harmonize 'file name' into 'filename'

[yarikoptic]

Text is using both 'file name' and 'filename' pretty much to the equal amount
ATM (see git grep outputs below). Code uses 'filename', and wikipedia has
https://en.wikipedia.org/wiki/Filename and prefers to use 'filename' in it. So
I decided to harmonize into 'filename'.

$> git grep  'file name' | grep '\.md' | grep -v MACRO | nl
	 1	src/02-common-principles.md:1.  **File extension** - a portion of the file name after the left-most
	 2	src/02-common-principles.md:are compulsory. For example a particular file name format is required when
	 3	src/02-common-principles.md:saved under a particular file name specified in the standard. This standard
	 4	src/02-common-principles.md:A file name consists of a chain of *entities*, or key-value pairs, a *suffix* and an
	 5	src/02-common-principles.md:`subject`, the file name MUST begin with the string `sub-<label>_ses-<label>`.
	 6	src/02-common-principles.md:If the `session` level is omitted in the folder structure, the file name MUST begin
	 7	src/02-common-principles.md:key/value pair MUST also be included as part of the file names themselves.
	 8	src/02-common-principles.md:produces a human readable file name, such as `sub-01_task-rest_eeg.edf`.
	 9	src/02-common-principles.md:It is evident from the file name alone that the file contains resting state
	10	src/02-common-principles.md:Entities within a file name MUST be unique.
	11	src/02-common-principles.md:For example, the following file name is not valid because it uses the `acq`
	12	src/02-common-principles.md:label, but must be included in file names (similarly to other key names).
	13	src/02-common-principles.md:meaning of file names and setting requirements on their contents or metadata.
	14	src/02-common-principles.md:to suppress warnings or provide interpretations of your file names.
	15	src/03-modality-agnostic-files.md:This file is REQUIRED if `sample-<label>` is present in any file name within the dataset.
	16	src/04-modality-specific-files/05-task-events.md:Where `<matches>` corresponds to task file name. For example:
	17	src/04-modality-specific-files/06-physiological-and-other-continuous-recordings.md:In the template file names, the `<matches>` part corresponds to task file name
	18	src/05-derivatives/02-common-data-types.md:is used to prevent clashing with the original file name.
	19	src/06-longitudinal-and-multi-site-studies.md:and [file names](02-common-principles.md#file-name-structure)
	20	src/99-appendices/03-hed.md:screen or the file name of the stimulus image.
	21	src/99-appendices/03-hed.md:       "LongName": "Stimulus file name",
	22	src/99-appendices/04-entity-table.md:[file name structure](../02-common-principles.md#file-name-structure),
	23	src/99-appendices/06-meg-file-formats.md:that not only the file names, but also the internal file pointers will be
	24	src/99-appendices/09-entities.md:[file name structure](../02-common-principles.md#file-name-structure).
	25	src/CHANGES.md:-   \[FIX] Clarify use of session entity in file names [#532](https://github.com/bids-standard/bids-specification/pull/532) ([Moo-Marc](https://github.com/Moo-Marc))
	26	src/CHANGES.md:-   \[FIX] Specify marker file names for KIT data (MEG) [#62](https://github.com/bids-standard/bids-specification/pull/62) ([monkeyman192](https://github.com/monkeyman192))
	27	src/CHANGES.md:-   Added missing `sub-<participant_label>` in behavioral data file names.
	28	src/pregh-changes.md:-   Added missing `sub-<participant_label>` in behavioral data file names.

$> git grep 'filename' | grep '\.md' | grep -v MACRO | nl
	 1	CONTRIBUTING.md:Make sure that all filename format templates, entity tables, and entity definitions are correct
	 2	src/02-common-principles.md:(with the same filename as the `.nii[.gz]` file, but with a `.json` extension).
	 3	src/03-modality-agnostic-files.md:      "filename": ("REQUIRED", "There MUST be exactly one row for each file."),
	 4	src/03-modality-agnostic-files.md:filename	acq_time
	 5	src/04-modality-specific-files/02-magnetoencephalography.md:which saves the MEG sensor coil positions in a separate file with two possible filename extensions  (`.sqd`, `.mrk`).
	 6	src/05-derivatives/01-introduction.md:    status. Any modification of raw files must use a modified filename that does
	 7	src/05-derivatives/01-introduction.md:    not conflict with the raw filename. Further, any files created as part of a
	 8	src/05-derivatives/01-introduction.md:    derivative dataset must not match a permissible filename of a valid raw
	 9	src/05-derivatives/01-introduction.md:    dataset. Stated equivalently, if any filename in a derivative dataset has a
	10	src/05-derivatives/01-introduction.md:-   Each Derivatives filename MUST be of the form:
	11	src/05-derivatives/01-introduction.md:    `source_entities` MUST be the entire source filename, with the omission of
	12	src/05-derivatives/01-introduction.md:    the source suffix and extension. One exception to this rule is filename
	13	src/05-derivatives/01-introduction.md:-   There is no prohibition against identical filenames in different derived
	14	src/05-derivatives/03-imaging.md:filename.
	15	src/99-appendices/04-entity-table.md:specification, and establishes a common order within a filename.
	16	src/99-appendices/08-coordinate-systems.md:The `scanner` coordinate system is implicit and assumed by default if the derivative filename does not define **any** `space-<label>`.
	17	src/99-appendices/11-qmri.md:filenames will remain the same; however, the optional metadata (third column) may
	18	src/CHANGES.md:-   \[SCHEMA] Use macro for filename templates in file collections appendix [#787](https://github.com/bids-standard/bids-specification/pull/787) ([tsalo](https://github.com/tsalo))
	19	src/CHANGES.md:-   \[FIX] Accidentally swapped Neuromag/Elekta/MEGIN cross-talk & fine-calibration filename extensions [#621](https://github.com/bids-standard/bids-specification/pull/621) ([hoechenberger](https://github.com/hoechenberger))
	20	src/CHANGES.md:-   \[INFRA] SCHEMA: Declare entities by concept names, add entity field for filename components [#616](https://github.com/bids-standard/bids-specification/pull/616) ([effigies](https://github.com/effigies))
	21	src/CHANGES.md:-   \[FIX] Common principles: Fix filename in inheritance principle [#261](https://github.com/bids-standard/bids-specification/pull/261) ([Lestropie](https://github.com/Lestropie))
	22	src/CHANGES.md:-   \[FIX] Example for IntendedFor was missing session indicator in the filename [#129](https://github.com/bids-standard/bids-specification/pull/129) ([yarikoptic](https://github.com/yarikoptic))
	23	src/schema/README.md:the entity tables, entity definitions, filename templates, and metadata tables.
	24	src/schema/README.md:-   `entities.yaml`: Entities (key/value pairs in folder and filenames).
	25	src/schema/README.md:This file contains a dictionary in which each entity (key/value pair in filenames) is defined.
	26	src/schema/README.md:they appear in filenames _and_ their full names.
	27	src/schema/README.md:For example, the key for the "Contrast Enhancing Agent" entity, which appears in filenames as `ce-<label>`,
	28	src/schema/README.md:since many entities (such as `ce`) have very short filename elements.
	29	src/schema/README.md:The `entity` field is the entity as it appears in filenames. For example, the `entity` for `ceagent` is `ce`.
	30	src/schema/README.md:Given that all entities appear in filenames, they should all be strings and the `type` field should always be `string`.
	31	src/schema/README.md:For example, `run` should have an index, so a valid key-value pair in a filename would be `run-01`.
	32	src/schema/README.md:Keys are the filenames (without file extensions),
	33	src/schema/README.md:-   `datatypes/*.yaml`: Files in the `datatypes` folder contain information about valid filenames within a given datatype.
	34	src/schema/README.md:    Each dictionary contains a list of suffixes, entities, and file extensions which may constitute a valid BIDS filename.
	35	src/schema/README.md:-   `entities.yaml`: This file simply defines the order in which entities, when present, MUST appear in filenames.
	36	src/schema/README.md:Each dictionary corresponds to a group of suffixes that have the same rules regarding filenames.
	37	src/schema/README.md:**NOTE**: The order in which entities appear in these dictionaries does not reflect how they should appear in filenames.
	38	src/schema/README.md:This file contains a list of entities in the order in which they must appear in filenames.

[Remi-Gau]

Not sure about amending src/CHANGES.md given that its content is generated from the title of the merged PRs, but that's a minor point I think.

[effigies]

If we don't want them reverted, we'll need to edit the pr titles.

[sappelhoff]

If we don't want them reverted, we'll need to edit the pr titles.

I just did that

[sappelhoff]

Thanks @yarikoptic