fixed a type in get_field_mapping in ParseMapping

cdplib/db_migration/ParseMapping.py

@@ -86,7 +86,7 @@ class ParseMapping:
             source_name_tag = self._source_name_tag
 
         if target_name_tag is None:
-            source_name_tag = self._target_name_tag
+            target_name_tag = self._target_name_tag
 
         assert(all([set([source_name_tag, target_name_tag]) <= set(d)
                     for d in self._mapping]))