fix(example/deps): update dependency cloud_firestore to ^4.15.6 (#344)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

example/pubspec.yaml

@@ -21,7 +21,7 @@ environment:
   sdk: ">=3.3.0 <4.0.0"
 
 dependencies:
-  cloud_firestore: ^4.15.5
+  cloud_firestore: ^4.15.6
   firebase_core: ^2.25.4
   firestore_cache:
     path: ../