fix argument indexing

good old index bug, off by one
this time forgot about parfait 1-indexing

test/vool/to_mom/send/test_send_simple_string.rb

@@ -21,6 +21,7 @@ module Vool
     def test_args_one_str
       assert_equal Mom::IntegerConstant,    @ins.next.arguments[0].right.known_object.class
       assert_equal 1,    @ins.next.arguments[0].right.known_object.value
+      assert_equal [:next_message, :arguments, 2],    @ins.next.arguments[0].left.slots
     end
   end
 end