Fix format specifier used in RCTUIManager

Summary:
Here, `reactTag` is an `NSNumber *` which uses `%@` as the format specifier. Newer versions of clang can warn or error on this. This change prevents that from happening.

See also #15402

Local builds with clang 5.
Closes https://github.com/facebook/react-native/pull/15414

Differential Revision: D5583703

Pulled By: javache

fbshipit-source-id: a6d0d2246725cd096b1f3cd062ab5924740ab76c

React/Modules/RCTUIManager.m

@@ -1421,7 +1421,7 @@ RCT_EXPORT_METHOD(setJSResponder:(nonnull NSNumber *)reactTag
   [self addUIBlock:^(__unused RCTUIManager *uiManager, NSDictionary<NSNumber *, UIView *> *viewRegistry) {
     _jsResponder = viewRegistry[reactTag];
     if (!_jsResponder) {
-      RCTLogError(@"Invalid view set to be the JS responder - tag %zd", reactTag);
+      RCTLogError(@"Invalid view set to be the JS responder - tag %@", reactTag);
     }
   }];
 }