Update release number on homepage (#4116)

* Update release number on homepage

* Update README.md

Co-authored-by: Chris Elion <chris.elion@unity3d.com>

Co-authored-by: Chris Elion <chris.elion@unity3d.com>

README.md

 
 ## Releases & Documentation
 
-**Our latest, stable release is `Release 2`. Click
-[here](https://github.com/Unity-Technologies/ml-agents/tree/release_2_docs/docs/Readme.md)
+**Our latest, stable release is `Release 3`. Click
+[here](https://github.com/Unity-Technologies/ml-agents/tree/release_3_docs/docs/Readme.md)
 to get started with the latest release of ML-Agents.**
 
 The table below lists all our releases, including our `master` branch which is