Fix a typo in the docstring of nb_index.

Objects/typeobject.c

@@ -6276,7 +6276,7 @@ static slotdef slotdefs[] = {
            slot_nb_inplace_true_divide, wrap_binaryfunc, "/"),
     NBSLOT("__index__", nb_index, slot_nb_index, wrap_unaryfunc,
            "__index__($self, /)\n--\n\n"
-           "Return self converted to an integer, if self is suitable"
+           "Return self converted to an integer, if self is suitable "
            "for use as an index into a list."),
     MPSLOT("__len__", mp_length, slot_mp_length, wrap_lenfunc,
            "__len__($self, /)\n--\n\nReturn len(self)."),