Docs should read `from tmx2dataframe import tmx2dataframe`

## issue
The demo in the docs will throw and error:

```
import tmx2dataframe
metadata, df = tmx2dataframe.read(...)
```

## Error:
` module 'tmx2dataframe' has no attribute 'read'`

## Fix:
The import should be changed to the below I guess:

`from tmx2dataframe import tmx2dataframe`