[Chapter 20] - Some youtube links point to videos that are not long available

The links below point to videos that are no longer available:

- https://fpy.li/20-3 --> https://www.youtube.com/watch?v=A9e9Cy1UkME
- https://fpy.li/20-12 --> https://www.youtube.com/watch?v=M8Z65tAl5l4

This is what the browser displays after I open the links:
<img width="857" height="332" alt="Image" src="https://github.com/user-attachments/assets/ab051f94-2946-491b-9163-c56204aa69ca" />