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

Open git-guides/git-overview.md and locate the GitHub Learning Lab course link shown in the issue. Check the current destination against the suggested GitHub Skills locations, update the URL to the correct course location, and verify that the rendered link resolves.

Scritto dal modello di indicizzazione a partire dal testo della issue.