[css-backgrounds-3] Update value definition syntax reference from CSS2 to css-values-3 now that we're using # multipliers. Fixes w3c#2209.

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -73,7 +73,8 @@ Contexts” in [[!CSS2]].
 <h2 id="values">
 Values and Interactions</h2>
 
-  This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]].
+  This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
+  using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
   Value types not defined in this specification are defined in CSS Values &amp; Units [[!CSS-VALUES-3]].
   Other CSS modules may expand the definitions of these value types.