drm/doc: fix VRR_ENABLED casing

Yes it's inconsitent with vrr_capable, but this is the actual uapi as
exercise by igt.

Fixes: ab7a664f ("drm: Document variable refresh properties")
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190130163006.28945-3-daniel.vetter@ffwll.ch