Update CHANGELOG.md for Database (firebase#261)

* Update CHANGELOG.md

* Update CHANGELOG.md

* Update CHANGELOG.md

* Update CHANGELOG.md

Author: schmidt-sebastian

Firebase/Database/CHANGELOG.md

@@ -1,3 +1,6 @@
+# v4.0.3
+- [fixed] Fixed a regression in v4.0.2 that affected the storage location of the offline persistent cache. This caused v4.0.2 to not see data written with previous versions.
+
 # v4.0.2
 - [fixed] Retrieving a Database instance for a specific `FirebaseApp` no longer returns a stale instance if that app was deleted.
 - [changed] Added message about bandwidth usage in error for queries without indexes.