Bug 14299: Today's checkouts not always sorting correctly

Bug 14299: Today's checkouts not always sorting correctly

Sometimes the today's checkouts do not sort correctly. This is due to a
simple typo in the comparison line where the bad key 'timstamp' is
compared against the correct key 'timestamp'.

Test Plan:
1) Check out a decent number of items in a row ( 5+ )
2) Hopefully you will see they are sorted incorrectly
3) Apply this patch
4) Reload the page
5) Note they are now sorted correctly

Followed test plan. Works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>