[css-box-3] Mark up box-edge keywords as grammar productions

Author: heycam

css-box-3/Overview.bs

@@ -263,7 +263,7 @@ Box-edge Keywords {#keywords}
 	</dl>
 
 	For convenience, the following value types are defined:
-	<pre>
+	<pre class="prod">
 		<dfn><<visual-box>></dfn> = content-box | padding-box | border-box
 		<dfn><<layout-box>></dfn> = content-box | padding-box | border-box | margin-box
 		<dfn><<paint-box>></dfn> = content-box | padding-box | border-box | fill-box | stroke-box