Fix typo in role

Author: Mauko Quiroga

openfisca_core/indexed_enums/enum.py

@@ -142,7 +142,7 @@ def encode(cls, array: Encodable) -> enums.EnumArray:
 
         .. versionchanged:: 35.5.0
             Fixed a bug when encoding :class:`bytes` arrays, now they're casted
-            to :str:`str` prior to encoding.
+            to :obj:`str` prior to encoding.
 
         .. versionchanged:: 35.5.0
             Fixed a bug when encoding scalar arrays of :class:`.Enum` items,