Update CONTRIBUTING.md (#4170)

we removed contributions welcome.

com.unity.ml-agents/CONTRIBUTING.md

 
 Lastly, if you're looking for input on what to contribute, feel free to reach
 out to us directly at ml-agents@unity3d.com and/or browse the GitHub issues with
-the `contributions welcome` label.
+the `Requests` or `Bug` label.
 
 ## Git Branches