Type hint should be str

[drewoldag]

I believe that in the function definition for create_resutls_dir, the type hint for postfix should be str.

https://github.com/lincc-frameworks/fibad/blob/main/src/fibad/config_utils.py#L237

Not 100% sure here, but looking at the way it's used, and the existing docstring, makes me think that we don't need to specify Union[Path, str] here.