Update README.md

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Updated the image link in the README to use the 'main' branch instead of 'master'.

📊 Key Changes

• Changed the image URL from https://github.com/ultralytics/mtc/blob/master/cover.jpg to https://github.com/ultralytics/mtc/blob/main/cover.jpg in the README file.

🎯 Purpose & Impact

• Correct URL Reference: Ensures the image used in the README is correctly sourced from the 'main' branch, enhancing document accuracy.
• Branch Standardization: Aligns with modern branch naming conventions, promoting inclusivity and clarity.