In examples of Oracle PL/SQL code, use varchar2 not varchar.

Oracle recommends using VARCHAR2 not VARCHAR, allegedly because they might
someday change VARCHAR to be spec-compliant about distinguishing null from
empty string.  (I'm not holding my breath, though.)  Our examples of PL/SQL
code were using VARCHAR, which while not wrong is missing the pedagogical
opportunity to talk about converting Oracle type names to Postgres.  So
switch the examples to use VARCHAR2, and add some text about what to do
with common Oracle type names like VARCHAR2 and NUMBER.  (There is probably
more to be said here, but those are the ones I'm sure about offhand.)
Per suggestion from rapg12@gmail.com.

Discussion: <20160521140046.22591.24672@wrigleys.postgresql.org>