drm: Fix subtle spelling error in drm_crtc_state

The drm_crtc_state documentation contains a subtle misspelling of the
word subtle. Correct it.
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

include/drm/drm_crtc.h

@@ -78,7 +78,7 @@ struct drm_plane_helper_funcs;
 /**
  * struct drm_crtc_state - mutable CRTC state
  *
- * Note that the distinction between @enable and @active is rather subtile:
+ * Note that the distinction between @enable and @active is rather subtle:
  * Flipping @active while @enable is set without changing anything else may
  * never return in a failure from the &drm_mode_config_funcs.atomic_check
  * callback. Userspace assumes that a DPMS On will always succeed. In other