Fix misplaced waitForPendingWrites (#3654)

* Fix misplaced waitForPendingWrites

* reformat

Author: wu-hui

Firestore/Source/API/FIRFirestore.mm

@@ -194,10 +194,6 @@ - (FIRWriteBatch *)batch {
                                          writeBatch:_firestore->GetBatch()];
 }
 
-- (void)waitForPendingWritesWithCompletion:(void (^)(NSError *_Nullable error))completion {
-  _firestore->WaitForPendingWrites(util::MakeCallback(completion));
-}
-
 - (void)runTransactionWithBlock:(id _Nullable (^)(FIRTransaction *, NSError **))updateBlock
                   dispatchQueue:(dispatch_queue_t)queue
                      completion:
@@ -318,6 +314,10 @@ - (void)shutdownWithCompletion:(nullable void (^)(NSError *_Nullable error))comp
   [self shutdownInternalWithCompletion:completion];
 }
 
+- (void)waitForPendingWritesWithCompletion:(void (^)(NSError *_Nullable error))completion {
+  _firestore->WaitForPendingWrites(util::MakeCallback(completion));
+}
+
 @end
 
 NS_ASSUME_NONNULL_END