typo addd

Author: affeldt-aist

HB/common/database.elpi

@@ -277,7 +277,7 @@ assert-building-bottom-up CurrentClass C3n C1n C2n :-
       Msg2 is "- declare an additional structure that inherits from both "
         ^ C1nS ^ " and " ^ C2nS
         ^ " and from which " ^ Before ^ " and/or " ^ After ^ " inherit.",
-      coq.error "You must declare the hierarchy bottom-up or addd a missing join."
+      coq.error "You must declare the hierarchy bottom-up or add a missing join."
        "There are two ways out:"
        Msg1
        Msg2)