Fix three links to the contributing docs for the sdk

Change-Id: I7da01e06279b649021c3c40e44c223cd1cec02c4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/405860
Reviewed-by: Samuel Rawlins <[email protected]>
Auto-Submit: Brian Wilkerson <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: bwilkerson

pkg/analysis_server/CONTRIBUTING.md

@@ -37,4 +37,4 @@ To run just the analysis server integration tests:
 
 
 [building]: https://github.com/dart-lang/sdk/wiki/Building
-[contributing]: https://github.com/dart-lang/sdk/wiki/Contributing
+[contributing]: https://github.com/dart-lang/sdk/blob/master/CONTRIBUTING.md

pkg/analysis_server_plugin/CONTRIBUTING.md

@@ -20,5 +20,5 @@ document][coding style].
 
 [building]: https://github.com/dart-lang/sdk/wiki/Building
 [coding style]: https://github.com/dart-lang/sdk/blob/main/pkg/analyzer/doc/implementation/coding_style.md
-[contributing]: https://github.com/dart-lang/sdk/wiki/Contributing
+[contributing]: https://github.com/dart-lang/sdk/blob/master/CONTRIBUTING.md
 [wiki]: https://github.com/dart-lang/sdk/wiki

pkg/linter/CONTRIBUTING.md

@@ -130,4 +130,4 @@ one above, the
 [linter issue tracker]: https://github.com/dart-lang/linter/issues
 [Linking a pull request]: https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue
 [coding style]: https://github.com/dart-lang/sdk/blob/main/pkg/analyzer/doc/implementation/coding_style.md
-[contributing]: https://github.com/dart-lang/sdk/wiki/Contributing
+[contributing]: https://github.com/dart-lang/sdk/blob/master/CONTRIBUTING.md