Update CONTRIBUTING.md

fix link

Author: SteveL-MSFT

.github/CONTRIBUTING.md

@@ -289,5 +289,5 @@ Once you sign a CLA, all your existing and future pull requests will be labeled
 [good-git-resources]: https://help.github.com/articles/good-resources-for-learning-git-and-github/
 [contribute-issues]: #contributing-to-issues
 [open-issue]: https://github.com/PowerShell/PowerShell/issues
-[help-wanted-issue]: https://github.com/powershell/powershell/issues?q=is%3Aopen+is%3Aissue+label%3AUp-for-Grabs
+[up-for-grabs]: https://github.com/powershell/powershell/issues?q=is%3Aopen+is%3Aissue+label%3AUp-for-Grabs
 [semantic linefeeds]: http://rhodesmill.org/brandon/2012/one-sentence-per-line/