Fixing two broken links

[horizon-blue]

Hi there. First of all, thanks for putting together such an awesome book!

While reading through Chapter 2 and 3 of probmods, I noticed that two of the URLs are broken:

• On Chapter 2 - Example: Intuitive Physics, the PDF file for Hamrick et al. (2011) no longer exists on MIT's website. So I'm replacing it with an URL from COGSCI.
• On Chapter 3 Exercise - Exercise 2: Conditioning and Intervention, the link back to Medical Diagnosis section in Chapter 2 is broken. This seems to be something that's supposed to be updated as part of the cleanup several years ago. Hopefully it isn't too late to fix it now :)

I've tested my changes on my local Jekyll session and verified that the website builds correctly.

(On an unrelated note: the description of this GitHub repo links to probmods.org/v2, which no longer exists. Perhaps it's time to update that link as well? :D)

[horizon-blue]

It seems like Chapter 5: Conditional Dependence does not have a corresponding reading page yet (so the link at the bottom of the Chapter also leads to a 404 page) :). I see that there's a file for this Chapter, but it needs to be converted to render on the website