Update Prior and Posterior Predictive Checks tutorial

[thomasaarholt]

There is a broken section on the Prior and Posterior Predictive Checks tutorial that is raising a python error.

Scroll most of the way down the page to see it.

It would be nice if someone could fix it, as it's a well-used reference. Additionally, I just had a dimension mismatch in my own code, which led me to that page. Any fix would be nice to see.

[OriolAbril]

I moved the issue back to pymc repo because while the post is a notebook, it is one of the core notebooks that are here in pymc repo.

[OriolAbril]

This has been fixed already so I'll close the issue, thanks for reporting @thomasaarholt