[css-overflow-3] Fix typo in example markup

Author: frivoal

css-overflow-3/Overview.bs

@@ -510,7 +510,7 @@ Scrollbars and Layout</h3>
 			</style>
 			<article>
 			  <aside>×</aside>
-			</section>
+			</article>
 		</xmp>
 
 		If no scrollbars are present on <code>&lt;article></code>,
@@ -543,7 +543,7 @@ Scrollbars and Layout</h3>
 			  <section>
 			    This section has big margins.
 			  </section>
-			</section>
+			</article>
 		</xmp>
 
 		If we assumed that <code>&lt;article></code> needed scrollbars,