Fixed typos (#159331)

Here's another one of my PRs where I hunt for typos across `flutter`
repo.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

docs/contributing/Style-guide-for-Flutter-repo.md

@@ -1079,7 +1079,7 @@ However, where possible avoid global constants. Rather than `kDefaultButtonColor
 
 ### Avoid abbreviations
 
-Unless the abbreviation is more recognizable than the expansion (e.g. XML, HTTP, JSON), expand abbrevations
+Unless the abbreviation is more recognizable than the expansion (e.g. XML, HTTP, JSON), expand abbreviations
 when selecting a name for an identifier. In general, avoid one-character names unless one character is idiomatic
 (for example, prefer `index` over `i`, but prefer `x` over `horizontalPosition`).
 

docs/ecosystem/testing/Plugin-Tests.md

@@ -195,7 +195,7 @@ See below for instructions on bringing up test scaffolding in a plugin (*does no
 1. Use of OCMock in new tests, especially Swift tests, is discouraged in favor of protocol-based
    programming and dependency injection. However, if your XCTests require OCMock, open the
    Package Dependencies section of the project in Xcode, and add the following dependency to
-   the RunnnerTests target:
+   the RunnerTests target:
 
    ```
    https://github.com/erikdoe/ocmock