[css-fonts-4] mark up an example as such

Author: svgeesus

css-fonts-4/Overview.bs

@@ -2110,13 +2110,15 @@ for user agents which don't support a particular font technology.
 	which do not support OpenType collections
 	will skip to the next url in the list. </p>
 
+	<div class="example">
 	<pre>
 	src: url(fonts/simple.woff);       /* load simple.woff relative to stylesheet location */
 	src: url(/fonts/simple.woff);      /* load simple.woff from absolute location */
 	src: url(fonts/coll.otc#foo);      /* load font foo from collection coll.otc
 	src: url(fonts/coll.woff2#foo);    /* load font foo from woff2 collection coll.woff2
 	src: url(fonts.svg#simple);        /* load SVG font with id 'simple' */
 	</pre>
+	</div>
 
 <h4 id="font-face-src-formats">Selecting items in the 'src!!descriptor'</h4>