doc: remove unnecessary INNER keyword

A join that was added in commit 9b2009c4 that did not use the INNER
keyword but the existing query used it.  It was cleaner to remove the
existing INNER keyword.

Reported-by: Peter Eisentraut

Discussion: https://postgr.es/m/a1ffbfda-59d2-5732-e5fb-3df8582b6434@2ndquadrant.com

Backpatch-through: 9.5

doc/src/sgml/pgbuffercache.sgml

@@ -149,7 +149,7 @@
 
 <screen>
 regression=# SELECT n.nspname, c.relname, count(*) AS buffers
-             FROM pg_buffercache b INNER JOIN pg_class c
+             FROM pg_buffercache b JOIN pg_class c
              ON b.relfilenode = pg_relation_filenode(c.oid) AND
                 b.reldatabase IN (0, (SELECT oid FROM pg_database
                                       WHERE datname = current_database()))