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 <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Laurent Pinchart <[e-mail]>

include/drm/drm_crtc.h

···
 /**
  * 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