[selectors-4] Fix URL of link to CSS Basic UI module (w3c#14002)

There was a typo in the link to CSS Basic UI 3 introduced recently
(`/` used in the domain name instead of `.`), see:
w3c@513d6f7#diff-ff7e41817bff012f3ddff1f611038e554675a63acfb0ae367398437958b0baf4R5397

The URL should also have a final `/` to avoid a redirect.

This update rather uses `[[CSS3UI inline]]` to let Bikeshed generate the URL
and avoid any mistake.

Author: tidoust

selectors-4/Overview.bs

@@ -5428,7 +5428,7 @@ Changes Since Level 3</h3>
 	<p>
 		<em>The following selectors
 		were previously in
-		<a href="https://www/w3/org/TR/css-ui-3">CSS Basic UI 3</a> 
+		[[CSS3UI inline]]
 		and have been moved to the main Selectors 4 specification 
 		for ease of reference.</em>
 	</p>