netfilter: Fix typo in NF_CONNTRACK Kconfig option description

The NF_CONNTRACK Kconfig option description makes an incorrect reference
to the "meta" expression where the "ct" expression would be correct.This
patch fixes the respective typographical error.

Fixes: d497c635 ("netfilter: add help information to new nf_tables Kconfig options")
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>