drm: Fix scaling_mode docs

Fix the bad copy-pasta in the scaling_mode docs.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210915142300.15631-1-ville.syrjala@linux.intel.com
Reviewed-by: Simon Ser <contact@emersion.fr>