[DOCS] for sample: Unit testing of Cloud Functions offline test not working

### Which sample?

**Sample name or URL** **Link to closest heading**
https://firebase.google.com/docs/functions/unit-testing

### What is the issue with this sample's docs?
Not working when copying 1 to 1. Always getting the following error when trying to do the offline sample:
`
Error: The default Firebase app does not exist. Make sure you call initializeApp() before using any of the Firebase services.`

Code not working