Use of PlanPro importer in example outdated/not working

The example uses `from planpro_importer.reader import PlanProReader` and
```Python
reader = PlanProReader("MVP") 
topology = reader.read_topology_from_plan_pro_file()
```

Importing the required functionality as `from planpro_importer import PlanProVersion, import_planpro` and using
```Python
topology = import_planpro("MVP", PlanProVersion.PlanPro19)
```
should work