[AKS] fix CredentialResult value format

[zqingqing1]

This checklist is used to make sure that common issues in a pull request are addressed. This will expedite the process of getting your pull request merged and avoid extra work on your part to fix issues discovered during the review process.

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[AutorestCI]

Automation for azure-sdk-for-python

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-python#3110

[azuresdkci]

Can one of the admins verify this patch?

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Automation for azure-sdk-for-node

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-node#3304

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Automation for azure-sdk-for-ruby

Nothing to generate for azure-sdk-for-ruby

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Automation for azure-sdk-for-go

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-go#2422