[css-color-4][editorial] Add missing wpt

Author: svgeesus

css-color-4/Overview.bs

@@ -623,6 +623,7 @@ Transparency: the 'opacity' property</h3>
 	</pre>
 
 	<wpt>
+		clip-opacity-out-of-flow.html
 		t32-opacity-basic-0.0-a.xht
 		t32-opacity-basic-0.6-a.xht
 		t32-opacity-basic-1.0-a.xht