[FIX] l10_be_invoice_bba: trigger the onchange when reference_type is modified

Bug introduced by during the module migration

https://github.com/odoo/odoo/commit/dc6e7ca25ba4f1b5b6ce9c1857527e14ff919e31

addons/l10n_be_invoice_bba/models/account_invoice.py

@@ -48,7 +48,7 @@ class AccountInvoice(models.Model):
             if mod == int(bbacomm[-2:]):
                 return True
 
-    @api.onchange('partner_id', 'type')
+    @api.onchange('partner_id', 'type', 'reference_type')
     def _onchange_partner_id(self):
         result = super(AccountInvoice, self)._onchange_partner_id()
         reference = False
@@ -62,7 +62,6 @@ class AccountInvoice(models.Model):
         self.reference = reference
         return result
 
-    @api.multi
     def generate_bbacomm(self, type, reference_type, partner_id, reference):
         reference = reference or ''
         algorithm = False