NavigatorIOS docs: need to import PropTypes

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Closes https://github.com/facebook/react-native/pull/9141

Differential Revision: D3650195

Pulled By: hramos

fbshipit-source-id: 2a4bb06f92109b36bec761259e13a5782cc9929b

Libraries/Components/Navigation/NavigatorIOS.ios.js

@@ -110,7 +110,7 @@ type Event = Object;
  * navigates to. `initialRoute` represents the first route in your navigator.
  *
  * ```
- * import React, { Component } from 'react';
+ * import React, { Component, PropTypes } from 'react';
  * import { NavigatorIOS, Text } from 'react-native';
  *
  * export default class NavigatorIOSApp extends Component {