[css-2015] Also replace 18.2 by css-color-3

Author: fantasai

css-2015/Overview.bs

@@ -118,8 +118,9 @@ Introduction {#intro}
 
 		<dt><a href="http://www.w3.org/TR/css3-color/">CSS Color Level 3</a>
 		<dd>
-			Replaces CSS2&sect;4.3.6 and CSS2&sect;14.1, defining an extended range of color values;
-			and introduces the 'opacity' property.
+			Replaces CSS2&sect;4.3.6, CSS2&sect;14.1, and CSS2&sect;18.2,
+			defining an extended range of color values.
+			Also introduces the 'opacity' property.
 
 		<dt><a href="http://www.w3.org/TR/css3-background/">CSS Backgrounds and Borders Level 3</a>
 		<dd>