Documentation: bonding: correct xmit hash steps

Correct xmit hash steps for layer3+4 as introduced by commit
49aefd13

 ("bonding: do not discard lowest hash bit for non layer3+4
hashing").

Signed-off-by: Jonathan Toppins <jtoppins@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>