Update UsingNavigators.md

Summary:
Improve code formatting
Closes https://github.com/facebook/react-native/pull/11019

Differential Revision: D4220686

Pulled By: mkonicek

fbshipit-source-id: 019c02ee98e632b7f48567a22ca8cabeddadd584

docs/UsingNavigators.md

@@ -153,9 +153,11 @@ export default class MyScene extends Component {
     return (
       <View>
         <Text>Current Scene: {this.props.title}</Text>
+        
         <TouchableHighlight onPress={this.props.onForward}>
           <Text>Tap me to load the next scene</Text>
         </TouchableHighlight>
+        
         <TouchableHighlight onPress={this.props.onBack}>
           <Text>Tap me to go back</Text>
         </TouchableHighlight>