video: Correct docs for video_index_to_colour()

This uses the private data of the video uclass, not the console uclass
(its child). Update the comment to avoid confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: a032e4b55ea ("video: Move console colours to the video uclass")
diff --git a/include/video.h b/include/video.h
index 6dc42d4..55e9f32 100644
--- a/include/video.h
+++ b/include/video.h
@@ -163,7 +163,7 @@
  * The caller has to guarantee that the color index is less than
  * VID_COLOR_COUNT.
  *
- * @priv	private data of the console device
+ * @priv	private data of the video device (UCLASS_VIDEO)
  * @idx		color index
  * Return:	color value
  */