Update changelog for logging fix. (#3707)

Author: Chris Elion

com.unity.ml-agents/CHANGELOG.md

@@ -23,6 +23,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
  - Environment subprocesses now close immediately on timeout or wrong API version. (#3679)
  - Fixed an issue in the gym wrapper that would raise an exception if an Agent called EndEpisode multiple times in the same step. (#3700)
  - Fixed an issue where exceptions from environments provided a returncode of 0. (#3680)
+ - Fixed an issue where logging output was not visible; logging levels are now set consistently (#3703).
 
 ## [0.15.0-preview] - 2020-03-18
 ### Major Changes