[css-fonts-4] Fix markup of the <family-name> production

Author: cdoublev

css-fonts-4/Overview.bs

@@ -239,7 +239,7 @@ Font family: the 'font-family!!property' property</h3>
 Syntax of <<family-name>>
 </h4>
 
-	<pre><l><<family-name>></l> = <<string>> | <<custom-ident>>+</pre>
+	<pre class=prod><l><<family-name>></l> = <<string>> | <<custom-ident>>+</pre>
 
 	Font family names other than generic families must either be given quoted as <<string>>s,
 	or unquoted as a sequence of one or more <<custom-ident>>.