Very minor reworking of error message (#1304)

Signed-off-by: Nathan McDougall <nmcdougall@tonkintaylor.co.nz>

pandera/api/pyspark/model.py

@@ -387,7 +387,7 @@ def _collect_fields(cls) -> Dict[str, Tuple[AnnotationInfo, FieldInfo]]:
             if not isinstance(field, FieldInfo):
                 raise SchemaInitError(
                     f"'{field_name}' can only be assigned a 'Field', "
-                    + f"not a '{type(field)}.'"
+                    + f"not a '{type(field)}'."
                 )
             fields[field.name] = (AnnotationInfo(annotation), field)