Fixed doc for vlc_demux.

include/vlc_demux.h

@@ -106,7 +106,7 @@ enum demux_query_e
      * reading everything (you should not use this to call es_out_Control)
      * if you don't know what to do with it, just IGNORE it, it is safe(r)
      * -1 means all group, 0 default group (first es added) */
-    DEMUX_SET_GROUP,            /* arg1= int            can fail */
+    DEMUX_SET_GROUP,            /* arg1= int, arg2=const vlc_list_t *   can fail */
 
     /* Ask the demux to demux until the given date at the next pf_demux call
      * but not more (and not less, at the precision available of course).