getsockopt documentation fixed

doc/zmq_getsockopt.txt

@@ -20,7 +20,7 @@ specified by the 'option_name' argument for the 0MQ socket pointed to by the
 'socket' argument, and store it in the buffer pointed to by the 'option_value'
 argument. The 'option_len' argument is the size in bytes of the buffer pointed
 to by 'option_value'; upon successful completion _zmq_getsockopt()_ shall
-modify the 'option_value' argument to indicate the actual size of the option
+modify the 'option_len' argument to indicate the actual size of the option
 value stored in the buffer.
 
 The following options can be retrieved with the _zmq_getsockopt()_ function:
@@ -222,7 +222,8 @@ EXAMPLE
 ----
 /* Retrieve high water mark into hwm */
 int64_t hwm;
-rc = zmq_getsockopt (socket, ZMQ_HWM, &hwm, sizeof hwm);
+size_t hwm_size = sizeof (hwm);
+rc = zmq_getsockopt (socket, ZMQ_HWM, &hwm, &hwm_size);
 assert (rc == 0);
 ----