netfilter: ebt_nflog: fix Kconfig typo

The help text should refer to nflog instead of ulog. Noticed by
Krzysztof Halasa <khc@pm.waw.pl>.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>