Resources/Rethinking_2/Chp_04.ipynb: The hyperlink in Code 4.26 is broken

[vanislekahuna]

The last line of the markdown cell under Code 4.26 reads as:

"You can read more details of PyMC here"

However, it seems that the website associated to the hyperlink no longer exists. Perhaps the introductory overview of page of PyMC might serve as a better replacement?

[yahrMason]

Thanks. I will add this to #234