Fix missing return in example

Summary:
Found this minor issue while reading the docs.

n/a

 [DOCS] [BUGFIX] [Libraries/Text/Text.js] - Add return to example
Closes https://github.com/facebook/react-native/pull/16752

Differential Revision: D6274215

Pulled By: hramos

fbshipit-source-id: ef735eb9179ab69d2ed1bc4a8b5e921d42d88fb0

Libraries/Text/Text.js

@@ -243,11 +243,13 @@ const viewConfig = {
  * ```javascript
  * class MyAppHeaderText extends Component {
  *   render() {
+ *     return (
  *       <MyAppText>
  *         <Text style={{fontSize: 20}}>
  *           {this.props.children}
  *         </Text>
  *       </MyAppText>
+ *     );
  *   }
  * }
  * ```