Fixes code examples in README

[ikrevem]

This pull request fixes some issues found in the two code examples in the README.

The main changes are:

• Replace exact and like with exact_schema and like_schema (otherwise an ImportError occurs)
• Initialize the Enum object (which is used to validate the user role) by directly passing the three values as arguments (instead of passing a list which causes a SchemaError)
• Add the description key to the valid examples in test_users_endpoint (otherwise the validation fails with a SchemaError reporting a missing key)

[codedawi]

Thank you!!