Fixes type-hinting for articulation properties in `from_files_cfg.py` (#384)

# Description

The declaration of `ArticulationPropertiesCfg` is not found in orbit. I
guess it should be `ArticulationRootPropertiesCfg`. This MR fixes the
typing.

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have run all the tests with `./orbit.sh --test` and they pass
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

source/extensions/omni.isaac.orbit/omni/isaac/orbit/sim/spawners/from_files/from_files_cfg.py

@@ -29,7 +29,7 @@ class FileCfg(RigidObjectSpawnerCfg):
     scale: tuple[float, float, float] | None = None
     """Scale of the asset. Defaults to None, in which case the scale is not modified."""
 
-    articulation_props: schemas.ArticulationPropertiesCfg | None = None
+    articulation_props: schemas.ArticulationRootPropertiesCfg | None = None
     """Properties to apply to the articulation root."""
 
     fixed_tendons_props: schemas.FixedTendonsPropertiesCfg | None = None