Update UsingAListView.md - Add flex:1 to View

Summary:
Add flex:1 to View's style prop, so the ListView can be scrolled.
Closes https://github.com/facebook/react-native/pull/10304

Differential Revision: D3993754

Pulled By: JoelMarcey

fbshipit-source-id: 23d73b23310a5b39dea4cf3c6f3af0bf2901480f

docs/UsingAListView.md

@@ -35,7 +35,7 @@ class ListViewBasics extends Component {
   }
   render() {
     return (
-      <View style={{paddingTop: 22}}>
+      <View style={{flex: 1, paddingTop: 22}}>
         <ListView
           dataSource={this.state.dataSource}
           renderRow={(rowData) => <Text>{rowData}</Text>}