Update prompt-optimization-cookbook.ipynb with formatting improvements

[rajpathak-openai]

Summary

Briefly describe the changes and the goal of this PR. Make sure the PR title summarizes the changes effectively.

Motivation

Why are these changes necessary? How do they improve the cookbook?

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[Copilot]

Pull Request Overview

This PR makes minor formatting improvements to the prompt optimization cookbook for GPT-5, standardizing heading levels and notebook metadata.

• Updates heading levels from ## to ### for consistent markdown formatting
• Standardizes notebook kernel metadata to use generic Python 3 configuration
• Resets execution count for better notebook presentation