3.0 correct link to ProGit book

- correct the link to `Progit` as it doesn't seems to be http://progit.org anymore.
- also correct the git clone command line as specified here : https://help.github.com/articles/fork-a-repo

if this is good to merge, let me know as I already prepare similar change for master branch.
it didn't worked for me on a windows machine. I can revert this change if you want.