Update readme: changed `AdUser` class to `User`

[IvanRamyk]

changed AdUser class to User that is actually present in the SDK and added the correct import line to the example

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[IvanRamyk]

Also, there are other places where AdUser is mentioned but I fixed only description that is related to my use case where I'm sure that it works

[stcheng]

@IvanRamyk thank you. Could you sign the CLA? We could then import this change!

[IvanRamyk]

@stcheng done :)

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[facebook-github-bot]

@stcheng has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@stcheng merged this pull request in facebook/facebook-python-business-sdk@59b514b270ab3b4fbea8d2229651e303252d3710.