Update format.yml

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced GitHub action triggers for formatting workflow.

📊 Key Changes

• GitHub action now triggers on all pull requests, regardless of the branch.
• Pushes are only considered for the 'main' branch instead of both 'main' and 'master'.

🎯 Purpose & Impact

• 🔁 Ensures consistent code format across all contributions, enhancing code quality.
• 🌐 Simplifies workflow by focusing on the 'main' branch, which is the default branch for most repositories.
• 🚀 Potentially speeds up integration by automatically formatting code from all PRs, reducing manual review time.