Arch: fix typo in ArchBuildingPart.py [skip-ci]
Arch: fix typo in ArchBuildingPart.py [skip-ci]

Found by user PMac in https://forum.freecadweb.org/viewtopic.php?f=3&t=51049
Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master