Update changelog for samples. (#5103) (#5106)

Author: surfnerd

com.unity.ml-agents/CHANGELOG.md

@@ -32,6 +32,7 @@ and this project adheres to
 ### Minor Changes
 #### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
 - Updated com.unity.barracuda to 1.3.2-preview. (#5084)
+- Added 3D Ball to the `com.unity.ml-agents` samples. (#5077)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - The `encoding_size` setting for RewardSignals has been deprecated. Please use `network_settings` instead. (#4982)
 - Sensor names are now passed through to `ObservationSpec.name`. (#5036)