correct link to ProGit book

 master    origin/master   ✚ correct link to ProGit book

correct link to ProGit book

 master    origin/master   ✚ correct link to ProGit book

Revert "correct link to ProGit book"

This reverts commit 5a2364837e277d99359ac6318a8bdff1b37bef5d.

correct link to `ProGit` book