[css-fonts-4][editorial] clean up invalid syntax for highlight-css

Author: svgeesus

css-fonts-4/Overview.bs

@@ -8615,7 +8615,7 @@ Serializing font-related at-rules</h3>
 	<div class=example id="ex-serialize-same-font-feature-values">
 		For example, the declaration:
 
-		<pre highlight-css>
+		<pre class="highlight-css">
 		/* Repeated declaration names, and multiple blocks of the same type*/
 		@font-feature-values foo {
 			@swash { pretty: 0; cool: 2; }