Fix typo in cipher_kt_mode_{cbc, ofb_cfb}() doxygen.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1406667866-14226-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8953
Signed-off-by: Gert Doering <gert@greenie.muc.de>