doc: Fix titles of some pg_buffercache functions.

As in commit 59d6c03956, use <function> rather than <structname> in
the <title> to be consistent with how other functions in this
module are documented.

Oversights in commits dcf7e1697b and 9ccc049dfe.

Author: Noboru Saito <noborusai@gmail.com>
Discussion: https://postgr.es/m/CAAM3qn%2B7KraFkCyoJCHq6m%3DurxcoHPEPryuyYeg%3DQ0EjJxjdTA%40mail.gmail.com
Backpatch-through: 18

doc/src/sgml/pgbuffercache.sgml

@@ -497,7 +497,7 @@
  </sect2>
 
  <sect2 id="pgbuffercache-pg-buffercache-evict-relation">
-  <title>The <structname>pg_buffercache_evict_relation()</structname> Function</title>
+  <title>The <function>pg_buffercache_evict_relation()</function> Function</title>
   <para>
    The <function>pg_buffercache_evict_relation()</function> function is very
    similar to the <function>pg_buffercache_evict()</function> function.  The
@@ -515,7 +515,7 @@
  </sect2>
 
  <sect2 id="pgbuffercache-pg-buffercache-evict-all">
-  <title>The <structname>pg_buffercache_evict_all()</structname> Function</title>
+  <title>The <function>pg_buffercache_evict_all()</function> Function</title>
   <para>
    The <function>pg_buffercache_evict_all()</function> function is very
    similar to the <function>pg_buffercache_evict()</function> function.  The