Improve CLI instructions for 2FA

[hectorsector]

Suggested by @aryansharma1323 in the community forum:

So I finished Introduction to GitHub course using Web UI and now doing it on CLI. In the section “Commit a file”, I was stuck on the error

remote: Invalid username or password.
fatal: Authentication failed for 'https://github.com/username/repository.git/'
despite having right username and password. Turned out I have two-factor authentication enabled which is why I couldn’t access. To find a way around this error, I went to the Developer Settings in the Settings section, generated a token from the “Personal Access Tokens” section, and used it as password in the CLI. By using that personal access token, I could log in and push the changes.

It would be great if the course had something that helped first-time users overcome this issue. Please let me know where I can suggest these changes to the course.