Categorical features parameters ipynb - Invalid notebook error

[FeFortti]

[dexhunter]

I added a fix #21, would be great if maintainers can review, thanks!

[fohrloop]

Also other notebooks won't show content on GitHub For example, this: https://github.com/catboost/tutorials/blob/master/python_tutorial.ipynb

As a workaround, one can either
a) Clone the repo and check the contents locally (for example in VS Code)
b) Use Colab. The address is

https://colab.research.google.com/github/catboost/tutorials/blob/master/<relative_path>

For example: https://colab.research.google.com/github/catboost/tutorials/blob/master/categorical_features/categorical_features_parameters.ipynb