docs(flutter_bloc): add trailing comma to BlocListener API docs (#4106

)

packages/flutter_bloc/lib/src/bloc_listener.dart

@@ -69,7 +69,7 @@ typedef BlocListenerCondition<S> = bool Function(S previous, S current);
 ///   },
 ///   listener: (context, state) {
 ///     // do stuff here based on BlocA's state
-///   }
+///   },
 ///   child: Container(),
 /// )
 /// ```