docs: networking: replace skb_hwtstamp_tx with skb_tstamp_tx

docs: networking: replace skb_hwtstamp_tx with skb_tstamp_tx

Tiny doc fix. The hardware transmit function was called skb_tstamp_tx
from its introduction in commit ac45f602ee3d ("net: infrastructure for
hardware time stamping") in the same series as this documentation.

Fixes: cb9eff097831 ("net: new user space API for time stamping of incoming and outgoing packets")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20211220144608.2783526-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>