In the tutorial, I see two styles of writing mutual information, namely `I(X: Y)` and `I(X; Y)`. It would be better if we are consistent here. Maybe we can use `I(X; Y)` because I often see people use `I(X; Y)`. Page 2 <img width="103" alt="image" src="https://user-images.githubusercontent.com/1214890/68397785-03cfb700-0174-11ea-83f0-d1614014e135.png"> Page 3 <img width="207" alt="image" src="https://user-images.githubusercontent.com/1214890/68397532-99b71200-0173-11ea-9336-d91854489bdc.png">