Make success_ml use the same format as DW::Template->render_template

The page passed to success_ml used to need a leading slash, but that
made it inconsistent with what DW::Template->render_template expected
(the .tt file without a leading slash). So make them consistent: that
will be less painful later
Merge pull request #774 from afuna/success_ml-consistent-with-render_template

Make success_ml use the same format as DW::Template->render_template