Update README.md to main branch

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced Accessibility to Waveform and Training Visualization Images 🖼️

📊 Key Changes

• Updated image URLs in the README.md to point from the master branch to the main branch.

🎯 Purpose & Impact

• Consistency and Clarity: Aligns with GitHub's naming convention changes, improving consistency across documentation.
• Accessibility: Ensures that users and developers can access visual resources without errors, enhancing the understanding of the project's capabilities.
• User Experience: Smoothens the navigation and resource fetching process, potentially increasing user engagement and satisfaction.