pgsql: Change a VACUUM manual page word from 'deleted' to 'expired', so

Log Message:
-----------
Change a VACUUM manual page word from 'deleted' to 'expired', so DELETE
and UPDATE are clearly covered by the term.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
vacuum.sgml (r1.43 -> r1.44)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/vacuum.sgml.diff?r1=1.43&r2=1.44)