[css-counter-styles-3][editorial] fix markup, fixes #7565

Author: tabatkins

css-counter-styles-3/Overview.bs

@@ -157,7 +157,9 @@ Defining Custom Counter Styles: the ''@counter-style'' rule</h2>
 
 	The general form of an ''@counter-style'' rule is:
 
-	<pre>@counter-style <<counter-style-name>> { <<declaration-list>> }</pre>
+	<pre class=prod>
+		@counter-style <<counter-style-name>> { <<declaration-list>> }
+	</pre>
 
 	<dfn>&lt;counter-style-name></dfn> is a <<custom-ident>>
 	that is not an <a>ASCII case-insensitive</a> match for ''list-style-type/none''.