pgsql: Use now() rather than 'now' in an example in the PL/PgSQL docs.

Log Message:
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Use now() rather than 'now' in an example in the PL/PgSQL docs. From David
Fetter and Ben Calvert.

Modified Files:
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pgsql/doc/src/sgml:
plpgsql.sgml (r1.58 -> r1.59)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.58&r2=1.59)