chore: update yt video url

Author: michallm

README.md

@@ -47,8 +47,7 @@
 ## Demo
 
 Click to watch the demo:
-
-[![ClassLab demo](https://github.com/michallm/classlab/blob/main/images/preview.png?raw=true)](https://www.youtube.com/watch?v=UlLlNjXtSz4)
+[![ClassLab demo](https://github.com/michallm/classlab/blob/main/images/preview.png?raw=true)](https://www.youtube.com/watch?v=nnGHIrSkrZQ)
 
 <!-- ABOUT THE PROJECT -->