fix link in Contributing.md

Author: ebobrow

Contributing.md

@@ -65,7 +65,7 @@ and get a better grasp on the execution flow.
 
 ## Hack!
 
-Here are some [good starting issues](https://github.com/rust-lang/rustfmt/issues?q=is%3Aopen+is%3Aissue+label%3Agood-first-issue).
+Here are some [good starting issues](https://github.com/rust-lang/rustfmt/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22).
 
 If you've found areas which need polish and don't have issues, please submit a
 PR, don't feel there needs to be an issue.