[css-lists-3] Fix formatting issue

cc @fantasai

Author: FremyCompany

css-lists-3/Overview.bs

@@ -847,7 +847,7 @@ The Implicit ''list-item'' Counter</h3>
 		  6. Third top-level item
 		</pre>
 		given markup such as
-		<pre>
+		<xmp>
 		  <ol>
 		    <li>First top-level item
 		    <li value=5>Second top-level item, value=5
@@ -863,7 +863,7 @@ The Implicit ''list-item'' Counter</h3>
 		        <li>Third second-level item, list start=3
 		    <li>Third top-level item
 		  </ol>
-		</pre>
+		</xmp>
 	</div>
 
 <h3 id="creating-counters">