Use stderr instead of stdout for MFA prompt
Use stderr instead of stdout for MFA prompt

Signed-off-by: Nick Turner <f27183276cbdd968cb09d16c8e66a12d1e4e9e4b@amazon.com>

kubernetes-sigs/aws-iam-authenticator,kubernetes-sigs/aws-iam-authenticator,kubernetes-sigs/aws-iam-authenticator