[ticket/12566] "Private messages" should be "Send pivate message"

PHPBB3-12566
[ticket/12566] Also changed for ucp_pm_viewmessage.php

PHPBB3-12566
Merge remote-tracking branch 'PayBas/ticket/12566' into develop-ascraeus

* PayBas/ticket/12566:
  [ticket/12566] Also changed for ucp_pm_viewmessage.php
  [ticket/12566] "Private messages" should be "Send pivate message"