Typo fixed in zmq_socket(3)

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

doc/zmq_socket.txt

@@ -225,8 +225,8 @@ Outgoing routing strategy:: Fan out
 ZMQ_HWM option action:: Drop
 
 
-ZMQ_SUB
-^^^^^^^
+ZMQ_XSUB
+^^^^^^^^
 Same as ZMQ_SUB except that you subscribe by sending subscription messages to
 the socket. Subscription message is a byte 1 (for subscriptions) or byte 0
 (for unsubscriptions) followed by the subscription body.