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

drivers/gpu/drm/drm_connector.c

@@ -1367,7 +1367,7 @@ EXPORT_SYMBOL(drm_mode_create_scaling_mode_property);
  *
  *	Absence of the property should indicate absence of support.
  *
- * "vrr_enabled":
+ * "VRR_ENABLED":
  *	Default &drm_crtc boolean property that notifies the driver that the
  *	content on the CRTC is suitable for variable refresh rate presentation.
  *	The driver will take this property as a hint to enable variable