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:
| 04080634 ("selftests/net: so_txtime multi-host support")
a help text stating the wrong default clock was added.

This patch fixes the help text.

Fixes: 04080634 ("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.deSigned-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/so_txtime.c

@@ -421,7 +421,7 @@ static void usage(const char *progname)
 			"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"