[css-regions-1] Fix cross reference bikeshed error

An update to a newer version of HTML is probably desirable,
but this is the minimal change that will fix the cross-ref error.

Author: frivoal

css-regions-1/Overview.bs

@@ -413,7 +413,7 @@ The 'flow-into' property</h3>
 	the default traversal order
 	of sequential navigation modes
 	(such as cycling through links,
-	see e.g. <a href="http://www.whatwg.org/specs/web-apps/current-work/multipage/editing.html#sequential-focus-navigation-and-the-tabindex-attribute"><code>tabindex</code></a> [[HTML40]]).
+	see e.g. <a href="http://www.whatwg.org/specs/web-apps/current-work/multipage/editing.html#sequential-focus-navigation-and-the-tabindex-attribute"><code>tabindex</code></a> [[HTML401]]).
 
 	A <a>named flow</a> needs to be associated
 	with a <a>region chain</a>