pgsql: Fix wording of section comparing triggers and rules; old wording

Log Message:
-----------
Fix wording of section comparing triggers and rules; old wording as
confusing.

Modified Files:
--------------
pgsql/doc/src/sgml:
rules.sgml (r1.51 -> r1.52)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/rules.sgml?r1=1.51&r2=1.52)