text: Title: Incorrect example in BooleanWidget docstring
  Body: The docstring in BooleanWidget in
  
  https://github.com/django-import-export/django-import-export/blob/e855fcf8ed43af4dd681435f516e9f50ee19613d/import_export/widgets.py#L119
  
  shows example usage with missing brackets (the `widget` kwarg must be instance not a class)