Update README.md code example

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced code clarity for model loading and export in YOLO iOS App documentation.

📊 Key Changes

• Refactored model loading and exporting code into separate steps.
• Improved readability by assigning the loaded model to a variable before exporting.

🎯 Purpose & Impact

• Purpose: To make the documentation easier to understand, especially for new users or developers.
• Impact: Enhances the clarity of the README, potentially reducing setup errors and improving user onboarding experience. 📈👨‍💻👩‍💻