selftests/bpf: Fix spelling mistake "tranmission" -> "transmission"

There are two spelling mistakes in output messages. Fix these.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201214223539.83168-1-colin.king@canonical.com