fixed twitter app running error (#153)

When I update my react-native-simple-router version, twitter app is not working.

Errors like below:

``` js
Unable to resolve module ../index
```

Author: JackPu

examples/twitter-example/index.js

@@ -1,7 +1,7 @@
 import React from "react";
 import { StyleSheet } from 'react-native';
 
-import Router from '../index';
+import Router from '../../index';
 
 import HomePage from './pages/HomePage';
 import BackButton from './components/BackButton';