[css-images-4] Fix abstract level number

Author: LeaVerou

css-images-4/Overview.bs

@@ -11,7 +11,7 @@ TR: https://www.w3.org/TR/css4-images/
 Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Editor: Lea Verou, Invited Expert, http://lea.verou.me/about, w3cid 52258
-Abstract: This module contains the features of CSS level 3 relating to the <<image>> type and replaced elements.
+Abstract: This module contains the features of CSS level 4 relating to the <<image>> type and replaced elements.
 	It includes and extends the functionality of CSS level 2 [[CSS21]] and in the previous level of this specification [[css3-images]].
 	The main extensions compared to [[css3-images]] are several additions to the <<image>> type, such as the ''image()'' notation, the ''element()'' notation, and conic gradients.