[css-images-3] Mark up <radial-gradient()> production.

Author: heycam

css-images-3/Overview.bs

@@ -503,7 +503,7 @@ Radial Gradients: the ''radial-gradient()'' notation {#radial-gradients}
 
 	The radial gradient syntax is:
 
-	<pre>
+	<pre class=prod>
 		<dfn>radial-gradient()</dfn> = radial-gradient(
 		  [ <<ending-shape>> || <<size>> ]? [ at <<position>> ]? ,
 		  <<color-stop-list>>