Fix syntax error

Summary:
It is not itemPositoning it is itemPositioning

I have test it on iOS and tvOS
Closes https://github.com/facebook/react-native/pull/15426

Differential Revision: D5591807

Pulled By: javache

fbshipit-source-id: 0ad0bc32012c63f93f6b1528cae46c6dcba56706

React/Views/RCTTabBar.m

@@ -157,7 +157,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithCoder:(NSCoder *)aDecoder)
 #endif
 }
 
-- (UITabBarItemPositioning)itemPositoning
+- (UITabBarItemPositioning)itemPositioning
 {
 #if TARGET_OS_TV
   return 0;