Fix 'launch_w90_minimal' example

Pylint currently (AFAICT, correctly) identifies the following issue in the example:

```
examples/workflows/launch_w90_minimal.py
  Line: 145
    pylint: no-member / Module 'aiida.orm.nodes.data.upf' has no 'UPFGROUP_TYPE' member (col 39)
  Line: 157
    pylint: no-member / Module 'aiida.orm.nodes.data.upf' has no 'UPFGROUP_TYPE' member (col 43)
```