Fix invalid notebook format in Assistants API overview

[damon-openai]

Summary

Fix invalid JSON in the examples/Assistants_API_overview_python.ipynb notebook.

Motivation

Somehow, the JSON for this notebook was invalid. This PR can be verified using either jq or by opening the notebook in Jupyter.

Before this PR, the notebook failed to open, and jq reported an error. After this PR, the notebook opens successfully and jq outputs formatted JSON.

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