pgsql: Revert thinko introduced into prefix_selectivity() by my recent

Log Message:
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Revert thinko introduced into prefix_selectivity() by my recent patch:
make_greater_string needs the < procedure not the >= one. Spotted by
Peter.

Modified Files:
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pgsql/src/backend/utils/adt:
selfuncs.c (r1.245 -> r1.246)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c?r1=1.245&amp;r2=1.246)