Update src/Chapter3.hs

Co-authored-by: Dmitrii Kovanikov <[email protected]>

src/Chapter3.hs

@@ -1029,7 +1029,7 @@ instance Append Gold where
 
 instance Append [a] where
     append :: [a]  -> [a] -> [a]
-    append l1 l2 = l1 ++ l2
+    append = (++)
 
 instance (Append a) => Append (Maybe a) where
     append :: Maybe a -> Maybe a -> Maybe a