netty-server section: add missing method call on Server

The current "unfiltered.netty.Server(8080).plan(hello).run()" returns

```
error: not enough arguments for method apply: (portBindings: List[unfiltered.netty.PortBinding], handlers: List[() => io.netty.channel.ChannelHandler], beforeStopBlock: () => Unit, chunkSize: Int, engine: unfiltered.netty.Engine)unfiltered.netty.Server in object Server.
Unspecified value parameters handlers, beforeStopBlock, chunkSize, ...
```

The proposed change will properly start the server on http://127.0.0.1:8080
Merge pull request #253 from lbarasti/patch-1

Docs - netty-server section: add missing method call on Server