Add missing XML ID attribute

Discussion: https://www.postgresql.org/message-id/dc813a6f-60d9-991f-eecd-675a0921de11@gmx.de

Author: petere

doc/src/sgml/pgbuffercache.sgml

@@ -273,7 +273,7 @@
   </para>
  </sect2>
 
- <sect2>
+ <sect2 id="pgbuffercache-usage-counts">
   <title>The <function>pg_buffercache_usage_counts()</function> Function</title>
 
   <para>