[FIX] stock_voucher: Fix wrong variable name.

closes #410

Signed-off-by: Nicolas Mac Rouillon <nmr@adhoc.com.ar>

stock_voucher/wizards/stock_immediate_transfer.py

@@ -23,7 +23,7 @@ def process(self):
             return {
                 'actions': [
                     res,
-                    picking.do_print_voucher(),
+                    pickings.do_print_voucher(),
                 ],
                 'type': 'ir.actions.act_multi',
             }