Link to penguins tutorial. (#3361)

README.md

 
 In addition to our own documentation, here are some additional, relevant articles:
 
-* [Unity AI - Unity 3D Artificial Intelligence](https://www.youtube.com/watch?v=bqsfkGbBU6k)
+* [ML-Agents Penguins tutorial](https://learn.unity.com/project/ml-agents-penguins)
 
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