message: fix the i18n key used when informing about a message coming from an unknown sender
diff: @@ -909,7 +909,7 @@ jsxc.xmpp = {
  if (chat.length === 0) {
  jsxc.notice.add({
  msg: $.t('Unknown_sender'),
  - description: $.t('You_received_a_message_from_an_unknown_sender') + ' (' + bid + ').'
  + description: $.t('You_received_a_message_from_an_unknown_sender_') + ' (' + bid + ').'
  }, 'gui.showUnknownSender', [bid]);
  }
  