fix typo
rest_firewall: update the contents according to the newest ryu

this patch corresponds to the following merging:

  https://github.com/osrg/ryu/commit/44815b73758ca1cf660819d17558e66919b71c56

and corresponds to the following request:

  http://sourceforge.net/mailarchive/message.php?msg_id=31785720

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>