[DOCS]: dev instructions use wrong option for upgrade

### What happened?

Instructions have `pip install -u` instead of `pip install -U`.

### Relevant log output

```shell
N/A
```

### How often does this bug happen?

Every time

### What version of the app are you using?

Latest

### Contributions

- [x] I have read the contribution guidelines

### Code of Conduct

- [x] I agree to follow WLAN Pi's Code of Conduct