fix the dfs function to properly work in DSSL$2$:

getting this error: $../../../$Library$/$Racket$/8\mathrm{.}12/$pkgs$/$dssl$2/$private$/$syntax$.$rkt:$150$:$7$: Failed test: dfs$\_$to$\_$list$($example$\_$graph$())($graph$.$rkt:$241$:$0)$

reason: an error occurred $($but shouldn't have$)$

details: application: not a procedure;

expected a procedure that can be applied to arguments

given: #