pgsql: Make pg_upgrade documentation refer to 9.0 instead of 8.4.

Log Message:
-----------
Make pg_upgrade documentation refer to 9.0 instead of 8.4.

Fujii Masao

Modified Files:
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pgsql/doc/src/sgml:
pgupgrade.sgml (r1.4 -> r1.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/pgupgrade.sgml?r1=1.4&r2=1.5)