Update Subclass Form in README to pass Reel::Websocket instead of Reel::Request

I'm trying celluloid/reel for the first time and was trying to get things set up given the examples. However I was getting errors using the example in the Subclass Form when calling:

```
sock << "Hello everyone out there in WebSocket land!"
```

I realized this is due to attempting to call `write` on a `Reel::Request` as opposed to a `Reel::Websocket`. It could very well be that I'm not doing things properly, but this adjustment seems to have done the trick for me.