Fix link to git pro book

[iglpdc]

The link to Chacon's book was broken. This fixes it.

[jdblischak]

This fix was already merged into the core branch. See PR #179 (I had originally submitted directly to gh-pages in PR #178, but was instructed to send it to core). What is the process for contributing to this repo? Are we supposed to send PRs against core or gh-pages?

Also, why was the actual URL changed? I think Chapter 6.2, which explains the GitHub Flow, is a better place to send contributors than Chapter 5.2, which covers more complex details.

[iglpdc]

Yes, you're right @jdblischak. I wasn't aware of the core branch. I agree that we need guidelines on how to contribute to this repo: it doesn't say anywhere that we have a core branch, so it's easy to assume that PRs go against gh-pages as with the other repos.