Fix format of the changelog for validation. (#4340)

Author: surfnerd

com.unity.ml-agents/CHANGELOG.md

@@ -6,7 +6,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to
 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
-## [1.3.0-preview] 2020-08-12
+## [1.3.0-preview] - 2020-08-12
 
 ### Major Changes
 #### com.unity.ml-agents (C#)