qemu-options.hx: correct spice options streaming-video default document value to 'off'

since f1d3e586, the code had changed the default value to 'off', so this patch
make document and code are consistent.
Signed-off-by: NLi Zhijian <lizhijian@cn.fujitsu.com>
Message-id: 1470024419-10886-1-git-send-email-lizhijian@cn.fujitsu.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

qemu-options.hx

@@ -1145,7 +1145,7 @@ Configure wan image compression (lossy for slow links).
 Default is auto.
 
 @item streaming-video=[off|all|filter]
-Configure video stream detection.  Default is filter.
+Configure video stream detection.  Default is off.
 
 @item agent-mouse=[on|off]
 Enable/disable passing mouse events via vdagent.  Default is on.