pgsql: Documentation fix: CREATE TRIGGER 'WHERE' -> 'WHEN' Selena

Log Message:
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Documentation fix: CREATE TRIGGER 'WHERE' -> 'WHEN'

Selena Deckelmann

Modified Files:
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pgsql/doc/src/sgml:
release-9.0.sgml (r2.18 -> r2.19)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-9.0.sgml?r1=2.18&r2=2.19)