drm: fix typo in margin connector properties docs

drm: fix typo in margin connector properties docs

This was pointed out by Ville and Pekka in their replies, but
forgot to apply the change properly before pushing. Sorry for
the noise!

Signed-off-by: Simon Ser <contact@emersion.fr>
Fixes: 409f07d353b3 ("drm: document connector margin properties")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230305103503.42619-1-contact@emersion.fr