[selectors-5] Fix closing tag (#13811)

Oversight in 7d05440.

Author: cdoublev

selectors-5/Overview.bs

@@ -482,7 +482,7 @@ Grammar</h2>
 
 	<pre class=prod>
 	<dfn>&lt;combinator></dfn> = '>' | '+' | '~' | [ '|' '|' ] | [ / <<wq-name>> / ]
-	</pr>
+	</pre>
 
 	<ul>
 	<li id="white-space">