Update documentation for data selection in survey_api

In the survey_api documentation https://github.com/unioslo/tsd-api-docs/blob/master/integration/survey-api.md , under "Key filtering", the
following is written:

""""
Only data:

```
?select=data
```

""""

This ends up returning "null". If switching `data` with `answers`, a JSON response with just the answer ("data") is returned.

I.e. switch `?select=data` with `?select=answers`