[css-ui-4] Bikeshed fix

Author: frivoal

css-ui-4/Overview.bs

@@ -34,7 +34,7 @@ This module describes CSS properties which enable authors
 to style user interface related properties and values.
 
 <a href="https://www.w3.org/TR/REC-CSS1#anchor-pseudo-classes">Section 2.1 of CSS1</a> [[CSS1]]
-and <a href="https://www.w3.org/TR/CSS2/ui.html">Chapter 18 of CSS2</a> [[CSS2]]
+and <a href="https://www.w3.org/TR/CSS2/ui.html">Chapter 18 of CSS2</a> [[CSS21]]
 introduced several user interface related properties and values.
 <a href="https://www.w3.org/TR/2000/WD-css3-userint-20000216">User Interface for CSS3 (16 February 2000)</a> introduced several new user interface related features.