[scikit-learn] why the modification in the df-idf formula?

Hi guys, I'd like to understand why sklearn's implementation of tf-idf is different from the standard textbook notation as described in the docs: https://scikit-learn.org/stable/modules/feature_extraction.html#tfidf-term-w... Do you have any reference that I could take a look at? I didn't manage to find them in the docs, maybe I missed something? Thank you! Best wishes Sole