修改错别字

我觉得这个版本的zguide翻译，质量挺不错的。谢谢你们的辛勤劳动！

我怀疑，ZeroMQ Guide中这段文字是不是有误：
>The PUB-SUB socket pair is asynchronous. The client does zmq_recv(), in a loop (or once if that's all it needs). Trying to send a message to a SUB socket will cause an error. Similarly, the service does zmq_send() as often as it needs to, but must not do zmq_recv() on a PUB socket.

这里：
>to a SUB socket

是否应为：
>through a SUB socket

呢？