Fix typos in doc (cdevents#177)

Signed-off-by: t-kikuc <tkikuchi07f@gmail.com>

continuous-integration.md

@@ -14,7 +14,7 @@ Continuous Integration (CI) events include the subject and predicates related to
 
 ## Subjects
 
-This specification defines three subjects in this stage: `builds`, `artifacts` and `tests`. Events associated with these subjects are typically generated either by a CI system that orchestrates the process or by a specific build or test tool directly. Some artifact events may be generated by the system that stores the artifact as well.
+This specification defines two subjects in this stage: `build` and `artifact`. Events associated with these subjects are typically generated either by a CI system that orchestrates the process or by a specific build or test tool directly. Some artifact events may be generated by the system that stores the artifact as well.
 
 | Subject | Description | Predicates |
 |---------|-------------|------------|
@@ -27,7 +27,7 @@ This specification defines three subjects in this stage: `builds`, `artifacts` a
 
 A `build` is a process that uses a recipe to produce an artifact from source code.
 
-__Note:__ The data model for `builds`, apart from `id` and `source`, only includes the identifier of the artifact produced by the build. The inputs to the build process are not specified yet.
+__Note:__ The data model for `build`, apart from `id` and `source`, only includes the identifier of the artifact produced by the build. The inputs to the build process are not specified yet.
 
 | Field | Type | Description | Examples |
 |-------|------|-------------|----------|

core.md

@@ -16,7 +16,7 @@ Core events are at the lower level of abstraction in the dictionary: they descri
 ## Subjects
 
 In the context of Continuous Delivery, a *pipeline* is the definition of a set of *tasks* that needs to be performed to build, test, package, release and deploy software artifacts.
-The definition of *pipelines* and *tasks* is an authoring process, and has no event associated to it. CDEvents identifies two [*subjects*](./spec/README.md), [`pipelineRun`](#pipelinerun) and [`taskRun`](#taskrun), which are the runtime counterparts of *pipelines* and *tasks*.
+The definition of *pipelines* and *tasks* is an authoring process, and has no event associated to it. CDEvents identifies two [*subjects*](spec.md#subject), [`pipelineRun`](#pipelinerun) and [`taskRun`](#taskrun), which are the runtime counterparts of *pipelines* and *tasks*.
 
 | Subject | Description | Predicates |
 |---------|-------------|------------|

source-code-version-control.md

@@ -14,7 +14,7 @@ Source Code Control events includes the subjects and predicates related to chang
 
 ## Subjects
 
-This specification defines two subjects in this stage: `repository` and `change`. Events associated with these subjects are triggered by actions performed by software developers or bots that provide useful automation for software developers.
+This specification defines three subjects in this stage: `repository`, `branch`, and `change`. Events associated with these subjects are triggered by actions performed by software developers or bots that provide useful automation for software developers.
 
 | Subject | Description | Predicates |
 |---------|-------------|------------|