Add missing semicolon in NativeModulesIOS.md

Summary:
` ConstantsconstantsToExport` in` EnumConstants` has an error. It is a missing a closing curly braces after `return`.

Do not need a test.
Closes https://github.com/facebook/react-native/pull/13911

Differential Revision: D5145977

Pulled By: shergin

fbshipit-source-id: 267ea7d69253d10aaef763879318824b6e02d2c5

docs/NativeModulesIOS.md

@@ -329,7 +329,7 @@ You can then define methods and export your enum constants like this:
 {
   return @{ @"statusBarAnimationNone" : @(UIStatusBarAnimationNone),
             @"statusBarAnimationFade" : @(UIStatusBarAnimationFade),
-            @"statusBarAnimationSlide" : @(UIStatusBarAnimationSlide) }
+            @"statusBarAnimationSlide" : @(UIStatusBarAnimationSlide) };
 };
 
 RCT_EXPORT_METHOD(updateStatusBarAnimation:(UIStatusBarAnimation)animation