pgsql: Fix a few errors in comments.

Log Message:
-----------
Fix a few errors in comments. Patch by Fujii Masao, plus the one in
visibilitymap.c by me.

Modified Files:
--------------
    pgsql/contrib/pg_standby:
        pg_standby.c (r1.23 -> r1.24)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_standby/pg_standby.c?r1=1.23&r2=1.24)
    pgsql/src/backend/access/heap:
        visibilitymap.c (r1.4 -> r1.5)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/visibilitymap.c?r1=1.4&r2=1.5)
    pgsql/src/backend/tcop:
        postgres.c (r1.567 -> r1.568)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c?r1=1.567&r2=1.568)