Update README.md to `main` branch

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Update to the requirements.txt link in the README.

📊 Key Changes

• Changed the branch name in the link from master to main for the requirements.txt file.

🎯 Purpose & Impact

• Clarity: Makes sure the link directs users to the correct and updated branch, improving access to the necessary setup information.
• Inclusivity: Aligns with broader industry efforts to use more inclusive language by adopting main as the default branch name.
• User Experience: Provides an uninterrupted experience for users setting up the tool, ensuring they can easily find and install required dependencies.

Emoji to lighten up the message: 🔄📃💡