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

[ivintoiu]

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