remove » and « from the text mail because they will be escaped

core/templates/altmail.php

 <?php
-print_unescaped($l->t("Hey there,\n\njust letting you know that %s shared »%s« with you.\nView it: %s\n\nCheers!", array($_['user_displayname'], $_['filename'], $_['link'])));
+print_unescaped($l->t("Hey there,\n\njust letting you know that %s shared %s with you.\nView it: %s\n\nCheers!", array($_['user_displayname'], $_['filename'], $_['link'])));
 ?>
 
 --