modules/git-basics.md: Version Control with Git course on Udacity is not directly accessible anymore

https://classroom.udacity.com/courses/ud123/ (see https://github.com/kottans/git-course/blob/master/modules/git-basics.md#:~:text=Version%20Control%20with%20Git ) is not directly accessible.

Suggested fix: find and link an altertive with a similar scope of learning

Open modules/git-basics.md and inspect the current Udacity link and its surrounding learning resources. Find an accessible alternative covering a similar Version Control with Git scope, verify that it is reachable, then update the link in the module; done means the replacement is accessible and appropriately scoped.

Written by the indexing model from the issue text.