net: fix up hash documentation

commit 61b905da ("net: Rename skb->rxhash to skb->hash")
didn't update the documentation, fix this up.

Cc: Tom Herbert <therbert@google.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>