Fix broken link to "Introduction to Github" training

[tehunter]

Overview

The "Introduction to GitHub Learning Lab course" link was broken and points to a 404 error. I replaced it with a link to the "Github Skills" landing page.

Questions

Is it ok to link to the GitHub Skills landing page, or should it point straight to the "Introduction to GitHub" skills repo?

Review

@brianamarie @DylanAStark @a-a-ron added the initial commit to this page.