netfilter: fix typo

Backport of r37821

Signed-off-by: Luka Perkov <luka@openwrt.org>
Signed-off-by: Vasilis Tsiligiannis <acinonyx@openwrt.gr>

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@37838 3c298f89-4303-0410-b956-a3cf2f4a3e73