drm/doc: Fix copy paste error in drm_crtc_funcs.destroy()

The function is about cleaning up CRTC resources, not plane resources,
fix this in docbook.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_crtc.h

@@ -472,7 +472,7 @@ struct drm_crtc_funcs {
 	/**
 	 * @destroy:
 	 *
-	 * Clean up plane resources. This is only called at driver unload time
+	 * Clean up CRTC resources. This is only called at driver unload time
 	 * through drm_mode_config_cleanup() since a CRTC cannot be hotplugged
 	 * in DRM.
 	 */