Updated URL for Scott Chacon's "Pro Git" book from progit.org to git-scm.com/book

Looks like Scott Chacon has moved his book from his own website to the main git site.
Merge pull request #1 from wilcoxst/update-Pro-Git-book-url

Updated website for Scott Chacon's "Pro Git" book from progit.org to git-scm.com/book