Fix incorrect variable name (Ch. 2)

In the example code showing how the sink shuts down workers in a parallel pipeline,

```C
void *control = zmq_socket (context, ZMQ_PUB);
zmq_bind (control, "tcp://*:5559");
…
//  Send kill signal to workers
s_send (controller, "KILL");
```

The socket is declared as `control` but later referred to as `controller`.