switch from using :focus to :focus-visible in the example on outlines (#12556)

Author: bkardell

css-ui-4/Overview.bs

@@ -393,11 +393,11 @@ Outline properties</h2>
 	(e.g. for entering text or selecting a button).
 
 	<div class="example">
-		For example, to draw a thick black line around an element when it has the focus,
-		and a thick red line when it is active,
-		the following rules can be used:
+		For example, to draw a thick black line around elements in the 
+		'':focus-visible'' state, and a thick red line when it is in the 
+		'':active'' state, the following rules can be used:
 		<pre><code class="lang-css">
-		:focus  { outline: thick solid black }
+		:focus-visible  { outline: thick solid black }
 		:active { outline: thick solid red }
 		</code></pre>
 	</div>