Requirement is not correct

[hehao9504]

In Lecture 2 notebook, load_diabetes used argument 'as_frame', which is available in scikit-learn version >= 0.23
Requirement.txt sets "scikit-learn=0.20.4", and include "sklearn==0.0"?

[kuleshov]

Sorry for this! If you do a PR that fixes this issue, I'm happy to merge it.