Doc: correct thinko in pg_buffercache documentation.

Access to this module is granted to the pg_monitor role, not
pg_read_all_stats.  (Given the view's performance impact,
it seems wise to be restrictive, so I think this was the
correct decision --- and anyway it was clearly intentional.)

Per bug #16279 from Philip Semanchuk.

Discussion: https://postgr.es/m/16279-fcaac33c68aab0ab@postgresql.org

doc/src/sgml/pgbuffercache.sgml

@@ -24,8 +24,8 @@
  </para>
 
  <para>
-  By default use is restricted to superusers and members of the
-  <literal>pg_read_all_stats</literal> role. Access may be granted to others
+  By default, use is restricted to superusers and members of the
+  <literal>pg_monitor</literal> role. Access may be granted to others
   using <command>GRANT</command>.
  </para>