docs: segmentation-offloads.txt: Correct TCP gso_types

Pretty minor: just SKB_GSO_TCP  -> SKB_GSO_TCPV4 and
                   SKB_GSO_TCP6 -> SKB_GSO_TCPV6.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: David S. Miller <davem@davemloft.net>