capture_sync(.h): fix api reference

The following parameters of sync_pipe_start(capture_options *capture_opts, capture_session *cap_session, struct _info_data *cap_data, void(*update_cb)(void)) are not documented:
  parameter 'cap_data'

Change-Id: I3733c51eddbe94e9c0d680dd8436265b7878d70c
Reviewed-on: https://code.wireshark.org/review/13545
Reviewed-by: Anders Broman <a.broman58@gmail.com>

capchild/capture_sync.h

@@ -48,6 +48,7 @@ struct _info_data;
  *
  *  @param capture_opts the options
  *  @param cap_session a handle for the capture session
+ *  @param cap_data a struct with capture info data
  *  @param update_cb update screen
  *  @return             TRUE if a capture could be started, FALSE if not
  */