Fix typo in enums module doc

Co-authored-by: Mahdi Ben Jelloul <[email protected]>

Author: Mauko Quiroga

openfisca_core/indexed_enums/__init__.py

@@ -18,7 +18,7 @@
 
         from openfisca_core.indexed_enums import *  # Bad
         from openfisca_core.indexed_enums.config import ENUM_ARRAY_DTYPE  # Bad
-        from openfisca_core.ndexed_enums.Enum import Enum  # Bad
+        from openfisca_core.indexed_enums.Enum import Enum  # Bad
 
     The previous examples provoke cyclic dependency problems, that prevents us
     from modularizing the different components of the library, so as to make