docs: segmentation-offloads.txt: Fix ref to SKB_GSO_TUNNEL_REMCSUM

The doc originally called it SKB_GSO_REMCSUM. Fix it.

Fixes: f7a6272b ("Documentation: Add documentation for TSO and GSO features")
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: David S. Miller <davem@davemloft.net>