Update ratchet document for session

[mbonneau]

This was copied over from ratchetphp/Ratchet#703 (originally reported by @mshamaseen)

In ratchet document for http://socketo.me/docs/sessions#wrapped it says wrapped by WsServer, whereas it's actually Wrap the WsServer not wrapped by it.

the wraps should be like this:

IoServer::factory( new HttpServer( new SessionProvider( new WsServer( new Chat() ), new Handler\MemcachedSessionHandler($memcached) ) ), 6502 );

please update the document as it take me a hole week to figure this out ..

[hayden-t]

maybe suggest a pull request