[selectors] Tweak markup.

Author: tabatkins

selectors/Overview.bs

@@ -1356,7 +1356,7 @@ Universal selector </h3>
 	<div class="example">
 		<ul>
 			<li>
-				<span class=css>*[hreflang|=en]</span> and <span class=css>[hreflang|=en]</span> are equivalent,
+				<css>*[hreflang|=en]</css> and <css>[hreflang|=en]</css> are equivalent,
 			<li>''*.warning'' and ''.warning'' are equivalent,
 			<li>''*#myid'' and ''#myid'' are equivalent.
 		</ul>