Update broken links (#2677)

docs/usage.md

 
 #### Step 2 - Register all of your screen components
 
-Every screen that you want to be able to place in a tab, push to the navigation stack or present modally needs to be registered. We recommend doing this in a central place, like [screens/index.js](https://github.com/wix/react-native-navigation/blob/master/example/src/screens/index.ios.js).
+Every screen that you want to be able to place in a tab, push to the navigation stack or present modally needs to be registered. We recommend doing this in a central place, like [screens/index.js](https://github.com/wix/react-native-navigation/blob/master/example/src/screens/index.js).
 
 > Note: Since your screens will potentially be bundled with other packages, your registered name must be **unique**! Follow a namespacing convention like `packageName.ScreenName`.
 
   screen: 'example.PushedScreen',
   title: 'Pushed Screen'
 });
-```
+```