[docs] Update changelog (#5308)

com.unity.ml-agents/CHANGELOG.md

 
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - Fixed a bug where --results-dir has no effect. (#5269)
+- Fixed a bug where old `.pt` checkpoints were not deleted during training. (#5271)
 
 ## [1.9.1-preview] - 2021-04-13
 ### Major Changes