[css-lists-3] Mark up <reversed-counter-name> production

Author: heycam

css-lists-3/Overview.bs

@@ -759,7 +759,7 @@ Automatic Numbering With Counters</h2>
 	A <a>reversed</a> counter is created with the ''reversed()'' [=functional notation=],
 	which is defined as follows:
 
-	<pre>
+	<pre class="prod">
 		<dfn><<reversed-counter-name>></dfn> = reversed( <<counter-name>> )
 	</pre>