Turn single backquotes to double backquotes in filters

[sciunto]

Description

See #4117

I applied a regexp and checked the diff. So far, it's applied only on the filters submodule.
I would like to have your feedback first.

I think it is worth it: better documentation, less "bad" examples for future contributions.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.
• Consider backporting the PR with @meeseeksdev backport to v0.14.x

[pep8speaks]

Hello @sciunto! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file skimage/filters/thresholding.py:

Line 163:80: E501 line too long (80 > 79 characters)

Comment last updated at 2019-09-01 10:24:45 UTC

[soupault]

@sciunto Since warning and error messages are not rendered, I suggest to keep there the previous versions. Otherwise, this is a clear improvement, 👍 .

[hmaarrfk]

Are these new spaces a thing?

[sciunto]

This is a fix to match the convention.

[hmaarrfk]

people just make up standards. I guess it does follow the numpydoc documentation
https://numpydoc.readthedocs.io/en/latest/format.html#docstring-standard

but I would have thought that there should be no space, like a python dictionary.