Update CHANGELOG.md (#280)

Author: schmidt-sebastian

Firebase/Database/CHANGELOG.md

@@ -1,5 +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.
+- [fixed] Fixed a crash in `FIRApp deleteApp` for apps that did not have active database instances.
 
 # v4.0.2
 - [fixed] Retrieving a Database instance for a specific `FirebaseApp` no longer returns a stale instance if that app was deleted.