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.
Then perhaps keep the current statement as it is and add below that if you are on widows and having issues using the git url you can use the https url.