Fix www.youtube.com hostname

[glenn-jocher]

Fix for #7241

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced YouTube source handling in YOLOv5 dataset streaming.

📊 Key Changes

• Expanded URL checks to include 'www.youtube.com' along with 'youtube.com' and 'youtu.be'.

🎯 Purpose & Impact

• 🎯 Purpose: Ensures that YouTube video URLs with 'www' are recognized as valid sources for streaming.
• 🔍 Impact: Users will experience improved flexibility and fewer errors when using YouTube URLs in their datasets. This will be particularly beneficial for applications relying on real-time video analysis or training models with varied YouTube video data.