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.

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[Flutter Style Guide]:
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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