Update report_user_by_email.py example to use get_user_by_email() method instead of json_api_call()

[MarkTripod-Duo]

Description

Update report_user_by_email.py example

Motivation and Context

Make use of the newly added get_user_by_email() duo_client_python method()

How Has This Been Tested?

Manual testing

Types of Changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation / Examples