drm: Fix plane SIZE_HINTS property docs

Fix the typos in the plane SIZE_HINTS kernel docs.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 9677547d

 ("drm: Introduce plane SIZE_HINTS property")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240418114218.9162-1-ville.syrjala@linux.intel.com
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>