selftests/futex: Fix spelling mistake "Succeffuly" -> "Successfully"

There is a spelling mistake in a ksft_exit_fail_msg() message. Fix it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250715130627.1907017-1-colin.i.king@gmail.com