specs/vhost-user: improve VHOST_SET_VRING_NUM documentation

"number of vrings" doesn't help me understand the purpose of this
message. My understanding is that it is rather the size of the queue (in
modern terms).
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

docs/specs/vhost-user.txt

@@ -364,7 +364,7 @@ Message types
       Equivalent ioctl: VHOST_SET_VRING_NUM
       Master payload: vring state description
 
-      Sets the number of vrings for this owner.
+      Set the size of the queue.
 
  * VHOST_USER_SET_VRING_ADDR