DOC: Use use recommended library over deprecated library
Importing `pandas.io.json.json_normalize` in pandas 1.0.1 gives the following warning

```
FutureWarning: pandas.io.json.json_normalize is deprecated, use pandas.json_normalize instead
```

This PR updates the documentation here https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.json_normalize.html to use the recommended approach.
Thanks @martinbjeldbak !