Update README.md to main branch

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Update to the link referencing requirements.txt in README.

📊 Key Changes

• The link to the requirements.txt file in the README has been updated from pointing to the "master" branch to the "main" branch.

🎯 Purpose & Impact

• Clarity and Correctness: Ensures the link in the documentation points to the current and correct branch, following the industry-wide shift from using "master" to "main" for the default branch name. 🔄
• User Experience: Improves user experience by preventing confusion or errors when trying to access the requirements.txt file. Users will be directed to the correct location without encountering broken links. 🛠️

This change, while small, contributes to a more inclusive and streamlined user experience.