Update Networking.md

I ran into an `undefined is not a constructor` when I tried initiating a WebSocket connection. I imported the WebSocket module like so:`import { WebSocket } from 'react-native'`. 

When I changed it to `import { WebSocketModule } from 'react-native'` it worked. So I see two possible solutions:

1. renaming the `WebSocketModule` to `WebSocket`
2. updating the docs

This PR is for facebook#2.

docs/Networking.md

@@ -106,6 +106,8 @@ request.send();
 React Native also supports [WebSockets](https://developer.mozilla.org/en-US/docs/Web/API/WebSocket), a protocol which provides full-duplex communication channels over a single TCP connection.
 
 ```js
+import { WebSocketModule } from 'react-native';
+
 var ws = new WebSocket('ws://host.com/path');
 
 ws.onopen = () => {