add changelog entry

Author: t-ober

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Allow hierarchic grid structure for JointGridContainer [#768](https://github.com/ie3-institute/PowerSystemDataModel/issues/768)
 
 ### Fixed
+- Fixed wrong rated power unit hint [#804](https://github.com/ie3-institute/PowerSystemDataModel/issues/804)
 
 
 ### Changed