haskell-pandoc-types: update override for ghc 8.4.x

pkgs/development/haskell-modules/configuration-ghc-8.4.x.nix

···
     });
 
   # https://github.com/jgm/pandoc-types/issues/37
-  pandoc-types = self.pandoc-types_1_17_4;
+  pandoc-types = self.pandoc-types_1_17_4_1;
 
 }