doc: Fix pg_buffercache_evict() title

Use <function> rather than <structname> in the <title> to be consistent
with how other functions in this module are documented. Also suffix the
function name with () for consistency.

Backpatch to v17 where pg_buffercache_evict was introduced.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAExHW5uKWH8CuZc9NCb8XxSQc6uzvACV0cScebm54kF763ERAw@mail.gmail.com
Backpatch-through: 17

Author: danielgustafsson

doc/src/sgml/pgbuffercache.sgml

@@ -365,7 +365,7 @@
  </sect2>
 
  <sect2 id="pgbuffercache-pg-buffercache-evict">
-  <title>The <structname>pg_buffercache_evict</structname> Function</title>
+  <title>The <function>pg_buffercache_evict()</function> Function</title>
   <para>
    The <function>pg_buffercache_evict()</function> function takes a buffer
    identifier, as shown in the <structfield>bufferid</structfield> column of