PEP 769: Fixed exception in a example code snippet. (#4273)

Fixed exception in a example code snippet.

peps/pep-0769.rst

@@ -189,7 +189,7 @@ The implementation of ``attrgetter`` is quite direct: it implies using
 
     try:
         value = getattr(obj, "name")
-    except (IndexError, KeyError):
+    except AttributeError:
         value = XYZ
 
 Note we cannot rely on using ``getattr`` with a default value, as it would