Fix typo in collators.py error message (#63)

src/smashed/mappers/collators.py

@@ -91,7 +91,7 @@ def _get_padding_value(self, field_name: str) -> Union[int, float]:
             return self.unk_fields_pad_id
         else:
             raise ValueError(
-                f"Must specify a padding value for field {field_name}"
+                f"Must specify a padding value for field {field_name} "
                 f"or provide a extra_fields_padding_id attribute to "
                 "the mapper to handle unrecognized fields"
             )