Fix generate-prdoc for single audiences (#6103)

`prdoc check -n NNNN` (used in CI) enforces that yaml lists only be used
for lists with length more than one.

Previously this script generated a list no matter how many audiences
were provided, which generated a broken PRDoc when that number was 1.

Reproducing the issue locally:
```
python .github/scripts/generate-prdoc.py --pr 6022 --bump patch --audience runtime_user --force
prdoc check -n 6022
```
Fails on master but passes with this change

Co-authored-by: Bastian Köcher <[email protected]>