pgsql: Minor PL/PgSQL doc tweak: use current_timestamp rather than now()

Log Message:
-----------
Minor PL/PgSQL doc tweak: use current_timestamp rather than now() in
an example function.

Modified Files:
--------------
pgsql/doc/src/sgml:
plpgsql.sgml (r1.78 -> r1.79)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.78&r2=1.79)