Update README.md with OpenAI link

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced PR Summary feature in Ultralytics Actions for clearer pull requests.

📊 Key Changes

• Updated the PR Summary feature description to emphasize the use of OpenAI GPT4 for generating PR summaries.

🎯 Purpose & Impact

• Purpose: To provide a refined description of the PR Summary tool, clarifying its functionality and the technology used.
• Impact: Users can expect clearer and more efficient PR summaries, potentially improving their workflow regarding code reviews and collaboration.