updated simple-untyped.k

Updated simple-untyped.k to work in the new unified-builtins2 branch.
@andreisf @traianf please review (it is almost identical with the one under samples/java-rewrite-engine)
In fact, this is a comment I added in the Java backend version of ```simple-untyped.k```. I think it's better not to put it in the tutorial. The problem is that the macro used to expand for-loop is written in a way that expects the first argument of ```for``` construct to have sort ```Stmts``` instead of ```Stmt```. This works in the maude backend but not Java backend.