fix mypy issue

src/rechunk_data/__init__.pyi

@@ -11,7 +11,7 @@ def _save_dataset(
     dset: xr.Dataset, file_name: Path, encoding: Dict[str, Any], engine: str
 ) -> None: ...
 def _rechunk_dataset(dset: xr.Dataset) -> Tuple[xr.Dataset, Dict[str, Any]]: ...
-def rechunk_dataset(dset: xr.Dataset) -> xr.dataset: ...
+def rechunk_dataset(dset: xr.Dataset) -> xr.Dataset: ...
 def rechunk_netcdf_file(
     input_path: os.PathLike,
     output_path: Optional[os.PathLike] = ...,