bug in citext's upgrade script for parallel aggregates

Seems there's a small error in the upgrade script for citext for 1.1
to 1.2 which will cause min(citext) not to be parallel enabled.

max(citext)'s combinefunc is first set incorrectly, but then updated
to the correct value. I assume it was meant to set the combine
function for min(citext) instead.

Fix attached. I've assumed that because we're still in beta that we
can get away with this fix rather than making a 1.3 version to fix the
issue.

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 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services