pgsql: Update wording of how to prevent pg_dump from affecting

Log Message:
-----------
Update wording of how to prevent pg_dump from affecting statistics
collection.

Modified Files:
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pgsql/doc/src/sgml/ref:
pg_dump.sgml (r1.108 -> r1.109)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_dump.sgml?r1=1.108&r2=1.109)