Update "type auth.Challenge" comment example code
Update "type auth.Challenge" comment example code

This interface was changed in a0fdfb9d4dd6e84703d155d9c1f3c72d52324814, but the comment wasn't ever updated to match.

Signed-off-by: Andrew "Tianon" Page <4b2b69742da531f1900c4c4748d7a0ecda59626a@gmail.com>

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