fix typo and missing line

[dmnkbckr]

There are a typo and a missing line that prevent the workflow for own datasets to work.

[Anne-Andresen]

minor typo no ?

Before: self.root_dir, nifti_file_name) for nifti_file_name in nifti_file_names if nifti_file_names.endswith('.nii')]

After: self.root_dir, nifti_file_name) for nifti_file_name in nifti_file_names if nifti_file_name.endswith('.nii')]