modules/git-collaboration.md: GitHub & Collaboration course on Udacity is not directly accessible anymore

https://classroom.udacity.com/courses/ud456 (see https://github.com/kottans/git-course/blob/master/modules/git-collaboration.md#:~:text=GitHub%20%26%20Collaboration ) is not directly accessible.

Suggested fix: find and link an altertive with a similar scope of learning