Clarify use of Flow props types in cli example

Summary:
This was introduced as part of a codemod a few months back. Hopefully this edit makes the example code clearer.
Closes https://github.com/facebook/react-native/pull/17189

Differential Revision: D6613378

Pulled By: hramos

fbshipit-source-id: da7263b3ce2b5c45d6e312807c88743fe10cd15d

local-cli/templates/HelloWorld/App.js

@@ -19,7 +19,8 @@ const instructions = Platform.select({
     'Shake or press menu button for dev menu',
 });
 
-export default class App extends Component<{}> {
+type Props = {};
+export default class App extends Component<Props> {
   render() {
     return (
       <View style={styles.container}>