Return statement missing for renderScene

Summary:
Explain the **motivation** for making this change. What existing problem does the pull request solve?
Example code for "Using Navigators" under "THE BASICS" does not work. renderScene needs to return the components, not just instantiate them.

**Test plan (required)**

I copy-paste and ran the code but did not get anything rendered. I added a return statement before the component which made it work.

Arrow functions need a return statement when supplying a { block of code }.
Closes https://github.com/facebook/react-native/pull/9161

Differential Revision: D3663200

Pulled By: hramos

fbshipit-source-id: a8732dd1098de7c8ea915f459adb3403a8168f19

docs/UsingNavigators.md

@@ -78,7 +78,7 @@ render() {
     <Navigator
       initialRoute={{ title: 'My Initial Scene', index: 0 }}
       renderScene={(route, navigator) => {
-        <MyScene title={route.title} />
+        return <MyScene title={route.title} />
       }}
     />
   );
@@ -112,6 +112,7 @@ export default class SimpleNavigationApp extends Component {
       <Navigator
         initialRoute={{ title: 'My Initial Scene', index: 0 }}
         renderScene={(route, navigator) =>
+          return (
             <MyScene
               title={route.title}
   
@@ -131,6 +132,7 @@ export default class SimpleNavigationApp extends Component {
                 }
               }}
             />
+          )
         }
       />
     )