doc: SKB_GSO_[IPIP|SIT] have been replaced

Those enum values don't exist anymore.

Fixes: 7e13318d ("net: define gso types for IPx over IPv4 and IPv6")
CC: Tom Herbert <tom@herbertland.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>