Fix missing space in label_attr

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -


``label_attr`` in ``checkbox_radio_label`` is missing a space with ``parent_label_class``