[keyvalut] Enhance code style for sample file

[lbhsos]

Description

Modernize to follow with up-to-date coding style for
sdk/keyvault/azure-keyvault-keys/samples/list_operations.py

• change format to f-string

Closes: Azure#6

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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Testing Guidelines

• Pull request includes test coverage for the included changes.

[ianychoi]

Looks great!

[ianychoi]

Thank you! Let's discuss how it reflects to original repo later