Git: Add the rest of a review cycle

**Which module(s) and week(s) does this change effect?**
Module(s): Git
Week(s): Week 1

When @frosas was teaching this class, he showed the reviewer side of a PR, made a comment, had a trainee address the comment and re-push. This is a great exercise and we should include it in the syllabus, or at least suggest it in the instructor notes.