Update release notes

Author: maryamziaa

com.unity.ml-agents/CHANGELOG.md

@@ -28,6 +28,7 @@ and this project adheres to
 - Added no-graphics-monitor. (#6014)
 
 #### ml-agents / ml-agents-envs
+- Upgrade grpcio version. ((#6208)
 - Update Installation.md (#6004)
 - Updated Using-Virtual-Environment.md (#6033)