bump release name readme (#5832)

Author: maryamhonari

docs/Readme.md

@@ -43,7 +43,7 @@ See our [ML-Agents Overview](ML-Agents-Overview.md) page for detailed
 descriptions of all these features.
 ## Releases & Documentation
 
-**Our latest, stable release is `Release 19`. Click
+**Our latest, stable release is `Release 20`. Click
 [here](https://github.com/Unity-Technologies/ml-agents/tree/release_20_docs/docs/Readme.md)
 to get started with the latest release of ML-Agents.**