Invalid GitHub Learning Lab link in git-guides/git-overview.md

## Summary
The link to __GitHub Learning Lab__ in `git-guides/git-overview.md` points to an invalid domain. Clicking it returns `NS_ERROR_UNKNOWN_HOST`

## Actual Behavior
`[Introduction to GitHub Learning Lab course](https://lab.github.com/githubtraining/introduction-to-github)` points to a non-existing domain

## Expected Behavior
The link should direct to the correct location for the course.

## Suggested
Update the URL to https://skills.github.com/, respectively https://github.com/skills/introduction-to-github