selftests/net: so_txtime: usage(): fix documentation of default clock

selftests/net: so_txtime: usage(): fix documentation of default clock

The program uses CLOCK_TAI as default clock since it was added to the
Linux repo. In commit:
| 2c234ba69cdd ("selftests/net: so_txtime multi-host support")
a help text stating the wrong default clock was added.

This patch fixes the help text.

Fixes: 2c234ba69cdd ("selftests/net: so_txtime multi-host support")
Cc: Carlos Llamas <cmllamas@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Acked-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20220502094638.1921702-3-mkl@pengutronix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>