howto: Adapt to new RefinementInterface

doc/HOWTO_IMPLEMENT_A_FUNCTOR.org

@@ -102,7 +102,7 @@ type family Weight (f :: * -> *) :: *
 type family F1     (f :: * -> *) :: *
 type family F3     (f :: * -> *) :: *
 
-class (Ord (F1 f), Ord (F3 f)) => RefinementInterface (f :: * -> *) where
+class (Ord (F1 f), Ord (F3 f)) => RefinementInterface f where
   init   :: F1 f -> [Label f] -> Weight f
   update :: [Label f] -> Weight f -> (Weight f, F3 f, Weight f)
 #+END_SRC