l2tp: adjust comments about L2TPv3 offsets

The "offset" option has been removed by
commit 900631ee

 ("l2tp: remove configurable payload offset").

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Acked-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>