Add AppRegistry to imports in example

Summary:
Missing import throws variable error
Closes facebook#8988

Differential Revision: D3611142

fbshipit-source-id: b50edf52cdf6c723abaa7bd021cf11ee5b4d874d

Author: greyvugrin

docs/UsingNavigators.md

@@ -51,7 +51,7 @@ Notice the `export default` in front of the component declaration. This will _ex
 
 ```javascript
 import React, { Component } from 'react';
-import { View, Text } from 'react-native';
+import { AppRegistry } from 'react-native';
 
 import MyScene from './MyScene';