ipv6: Fix typo in net/ipv6/Kconfig

Two is used in the wrong context here, as you are connecting to an
IPv6 network over IPv4; not connecting two IPv6 networks to an IPv4
one.

Signed-off-by: Michael Beasley <youvegotmoxie@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>