web stories tutorial: broken link

## 📖 Missing or Out-of-Date Documentation

### Problem
link is broken. leads to a 404.

### Location
https://amp.dev/documentation/guides-and-tutorials/start/visual_story/setting_up/?format=stories

Paragraph _Step 1. Download the code_, last sentence _The completed version of the story can be found in the pets-completed.html file_.

The link to pets-completed.html is broken.