net: doc: Update references to skb->rxhash

In commit 61b905da

 ("net: Rename skb->rxhash to skb->hash"), skb->rxhash
was renamed to skb->hash. Update references in Documentation
accordingly.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>