Update lamba trigger order for Custom Auth Challenges

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Came up as part of reviewing[ Issue #14067 on JS repo](https://github.com/aws-amplify/amplify-js/issues/14067), but the lambdas should align conceptually with what is in Cognito's docs for order [here](https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-create-auth-challenge.html).

**URL page where content issue is**:
https://docs.amplify.aws/react/build-a-backend/functions/examples/custom-auth-flows/