Correct parameters in on_recv_stream callback

As per http://zeromq.github.io/pyzmq/api/generated/zmq.eventloop.zmqstream.html#zmq.eventloop.zmqstream.ZMQStream.on_recv_stream
the signature is 

    callback(stream, msg)

However the example gives

    def echo(msg, stream):
        stream.send_multipart(msg)