The maximum resolution encouraged in the help text for the qtcapture
arguments is 1280x480, which doesn't make sense. The patch author
intended to imply 720p but appears to have forgotten the height
dimension. In addition, clean up the help text a bit.

--qtcapture-height works with 720, anyway, so it's just a doc issue.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>