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:
| 040806343bb4 ("selftests/net: so_txtime multi-host support")
a help text stating the wrong default clock was added.

This patch fixes the help text.

Fixes: 040806343bb4 ("selftests/net: so_txtime multi-host support")
Cc: Carlos Llamas <[e-mail]>
Cc: Willem de Bruijn <[e-mail]>
Signed-off-by: Marc Kleine-Budde <[e-mail]>
Acked-by: Willem de Bruijn <[e-mail]>
Reviewed-by: Carlos Llamas <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

tools/testing/selftests/net/so_txtime.c

···
 			"Options:\n"
 			"  -4            only IPv4\n"
 			"  -6            only IPv6\n"
-			"  -c <clock>    monotonic (default) or tai\n"
+			"  -c <clock>    monotonic or tai (default)\n"
 			"  -D <addr>     destination IP address (server)\n"
 			"  -S <addr>     source IP address (client)\n"
 			"  -r            run rx mode\n"