Fixup internal links in `CONTRIBUTING.md` (#2552)

This is a follow on PR from https://github.com/flutter/samples/pull/2532

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [ ] I have added sample code updates to the [changelog].
- [ ] I updated/added relevant documentation (doc comments with `///`).


If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md
[changelog]: ../CHANGELOG.md

CONTRIBUTING.md

@@ -7,7 +7,7 @@ Do you work at Google? Great! You should **definitely** read this page before su
   
 * [Before you contribute]
 * [Contribute enhancements and fixes]
-* [Write a new sample]
+* [Writing a new sample]
 * [Sample checklist]
 
 _See also: [Flutter's code of conduct]_
@@ -260,7 +260,7 @@ one above, the [Software Grant and Corporate Contributor License Agreement].
 [Flutter's code of conduct]: https://github.com/flutter/flutter/blob/master/CODE_OF_CONDUCT.md
 [Before you contribute]: https://github.com/flutter/samples/blob/main/CONTRIBUTING.md#before-you-contribute
 [Contribute enhancements and fixes]: https://github.com/flutter/samples/blob/main/CONTRIBUTING.md#contribute-enhancements-and-fixes
-[Write a new sample]: https://github.com/flutter/samples/blob/main/CONTRIBUTING.md#writing-a-new-sample
+[Writing a new sample]: https://github.com/flutter/samples/blob/main/CONTRIBUTING.md#writing-a-new-sample
 [Sample checklist]: https://github.com/flutter/samples/blob/main/CONTRIBUTING.md#samples-checklist
 [file headers]: https://github.com/flutter/samples/blob/main/CONTRIBUTING.md#file-headers
 [Software Grant and Corporate Contributor License Agreement]: https://developers.google.com/open-source/cla/corporate