bpf: Fix spelling mistake "arithmatic" -> "arithmetic" in test_verifier

There are a couple of spelling mistakes in two literal strings, fix them.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200331100030.41372-1-colin.king@canonical.com