Update authentication details in `Using the push_to_hub API`

In [4. Sharing models and tokenizers - Sharing pretrained models - Using the push_to_hub API](https://huggingface.co/learn/nlp-course/chapter4/3?fw=pt#using-the-pushtohub-api), when the login inside a notebook is explained, it says:

```
In both cases, you should be prompted for your username and password
```

Instead of asking for username and password, it asks for the authentication token.