[css-ui-4] bikeshed fix

Author: frivoal

css-ui-4/Overview.bs

@@ -182,7 +182,7 @@ but has been moved to [[CSS-OVERFLOW-3#text-overflow]].
 
 <h3 id="values">Value Definitions</h3>
 
-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 [[!CSS21]]
 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]].
 Combination with other CSS modules may expand the definitions of these value types.