[css-overflow-3] Complete an example, where too much was merely implied

Author: frivoal

css-overflow-3/Overview.bs

@@ -505,7 +505,10 @@ Scrollbars and Layout</h3>
 
 		<xmp highlight=html>
 			<style>
-			  article { background: top right no-repeat url(circle.png); }
+			  article {
+			    background: top right no-repeat url(circle.png);
+			    position: relative;
+			    overflow: auto; }
 			  aside { position: absolute; top: 0; right: 0; }
 			</style>
 			<article>