[css-lists-3] Mark up counter function productions

Author: heycam

css-lists-3/Overview.bs

@@ -1351,7 +1351,7 @@ Outputting Counters: the ''counter()'' and ''counters()'' functions</h3>
 	whose [=used values=] represent counter values as strings or images.
 	They are defined as follows:
 
-	<pre>
+	<pre class="prod">
 		<dfn><<counter>></dfn> = <<counter()>> | <<counters()>>
 		<dfn>counter()</dfn>  =  counter( <<counter-name>>, <<counter-style>>? )
 		<dfn>counters()</dfn> = counters( <<counter-name>>, <<string>>, <<counter-style>>? )