Update README.md

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Updated a URL in the README file to fix a broken link to the Apple App Store badge.

📊 Key Changes

• 🔗 Corrected the URL for the Apple App Store badge image from the master branch to the main branch in the README file.

🎯 Purpose & Impact

• 🚀 Enhanced User Experience: Ensures that users can see the App Store badge correctly, making navigation smoother.
• 🔧 Maintenance and Accuracy: Keeps documentation up-to-date with current repository branch naming conventions.