Update CHANGELOG.md for #4274 (#4308) (#4314)

* Update CHANGELOG.md
Co-authored-by: andrewcoh <54679309+andrewcoh@users.noreply.github.com>

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

com.unity.ml-agents/CHANGELOG.md

 ### Major Changes
 #### com.unity.ml-agents (C#)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
-- The minimum supported python version for ml-agents-envs was changed to 3.6.1. (#4244)
+- The minimum supported Python version for ml-agents-envs was changed to 3.6.1. (#4244)
 - The interaction between EnvManager and TrainerController was changed; EnvManager.advance() was split into to stages,
 and TrainerController now uses the results from the first stage to handle new behavior names. This change speeds up
 Python training by approximately 5-10%. (#4259)
 calls to `StatsRecorder.Add()` with the same key in the same step will no
 longer overwrite each other. (#4236)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
+- The versions of `numpy` supported by ml-agents-envs were changed to disallow 1.19.0 or later. This was done to reflect
+a similar change in TensorFlow's requirements. (#4274)
 - Model checkpoints are now also saved as .nn files during training. (#4127)
 - Model checkpoint info is saved in TrainingStatus.json after training is concluded (#4127)
 - CSV statistics writer was removed (#4300).