][next] selftests: nettest: fix spelling mistake: "potocol" -> "protocol"

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

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>