[FIX] res_currency_print_on_check: issue with the xpath.

fix for https://github.com/OCA/l10n-canada/issues/8

When the xpath was first built only one form was in base.view_currency_form. Then the form for the search feartures had been added.  The second form made the xpath to return two fields. To avoid this behaviour, the form to be found in the xpath had been specified. It is less environment agnostic but base.view_currency_form form name seems to be stable.