update wcag link (#13147)

Author: alisonmaher

css-color-adjust-1/Overview.bs

@@ -130,7 +130,7 @@ Preferred Color Schemes {#preferred}
 	pairing default or <<system-color>> colors with author-specified colors
 	cannot guarantee any particular contrast level;
 	it might be necessary to set both foreground and background colors together
-	to ensure legibility [[WCAG21]].
+	to ensure legibility [[WCAG22]].
 
 	To enable pages to adapt to the user's [=preferred color scheme=],
 	user agents will match the '@media/prefers-color-scheme' media query