Update README.md

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Update the README link to the updated App Store image source.

📊 Key Changes

• 🔗 Updated the image source link in the README file from https://raw.githubusercontent.com/ultralytics/assets/master/app/app-store.svg to https://raw.githubusercontent.com/ultralytics/assets/main/app/app-store.svg.

🎯 Purpose & Impact

• ✅ Accuracy: Ensures the App Store image displays correctly by pointing to the current source.
• 🛡 Maintenance: Reflects the latest repository structure, improving future maintenance and clarity.
• 📞 Communication: Provides a better user experience by ensuring the README documentation is accurate and up-to-date.