Fix websocket example - ByteBufferList.clear() doesn't compile

Created by: rrbrambley

I copy/pasted the websocket example and get a compilation error:

@Override
public void onDataAvailable(DataEmitter emitter, ByteBufferList byteBufferList) {
    System.out.println("I got some bytes!");
    // note that this data has been read
    byteBufferList.clear();
}