selftests/seccomp: Fix spelling mistake "Coud" -> "Could"

There is a spelling mistake in an error message. Fix it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220504155535.239180-1-colin.i.king@gmail.com