Fix invoice scenario for new payment wizard state

tests/scenario_account_payment_invoice.rst

@@ -75,7 +75,7 @@ Partially pay line::
     >>> Payment = Model.get('account.payment')
     >>> pay_line = Wizard('account.move.line.pay', [line_to_pay])
     >>> pay_line.form.journal = payment_journal
-    >>> pay_line.execute('pay')
+    >>> pay_line.execute('start')
     >>> payment, = Payment.find()
     >>> payment.amount = Decimal('20')
     >>> payment.click('approve')