Wrong folder referenced

First two lab commands are these

```
git clone https://github.com/googlecodelabs/user-authentication-with-iap.git
cd iap-codelab
```

the first creates the `user-authentication-with-iap` folder
so, probably, the second has to be

`cd user-authentication-with-iap`

...and the same stands also for subsequent steps, e.g. `cd ~/iap-codelab/2-HelloUser` is wrong