[css-values-4] Fix types in css-values-4 draft (#5456)

Author: sottar

css-values-4/Overview.bs

@@ -2154,7 +2154,7 @@ Combination of <<color>></h4>
 	(that is, colors for which the red, green, and blue components specified
 	have been multiplied by the alpha).
 
-	<a>Addition</a> of <<number>> is likewise defined as
+	<a>Addition</a> of <<color>> is likewise defined as
 	the independent <a>addition</a> of each component
 	as a <<number>>
 	in premultiplied space.