[FIX] mass_mailing: typo that broke ensure_one down the line

Sometimes it crashed when the domain used for the mailing cron
returned more than one record

source commit: 1738f9c8f7

closes odoo/odoo#67556

X-original-commit: cb97f7c64a1fa575799a7832cb52e433436581e0
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mass_mailing/models/mailing.py

@@ -737,7 +737,7 @@ class MassMailing(models.Model):
 
             mail_values = {
                 'subject': _('24H Stats of %(mailing_type)s "%(mailing_name)s"',
-                             mailing_type=self._get_pretty_mailing_type(),
+                             mailing_type=mailing._get_pretty_mailing_type(),
                              mailing_name=mailing.subject
                             ),
                 'email_from': user.email_formatted,