selftests/net: Fix various spelling mistakes in TCP-AO tests

There are a handful of spelling mistakes in test messages in the
TCP-AIO selftests. Fix these.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Dmitry Safonov <dima@arista.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>