Add emacs to demo.nix

demo.nix

@@ -4,5 +4,5 @@ let
   ghotl-repl = (import ./ghotl-repl.nix { inherit pkgs; });
 in mkShell {
   name = "ghotl";
-  buildInputs = [ ghotl ghotl-repl ];
+  buildInputs = [ emacs ghotl ghotl-repl ];
 }