Navigator wasn't imported

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Closes https://github.com/facebook/react-native/pull/9556

Differential Revision: D3764387

fbshipit-source-id: 71cc80eff351298c1b3ac5a134191a7b64ea649c

docs/UsingNavigators.md

@@ -28,7 +28,7 @@ For simplicity's sake, let's define a simple scene that displays a bit of text.
 
 ```javascript
 import React, { Component } from 'react';
-import { View, Text } from 'react-native';
+import { View, Text, Navigator } from 'react-native';
 
 export default class MyScene extends Component {
   static get defaultProps() {