[ticket/12563] Changes two style language variable for better understanding

Little changes on two language variables. 

PHPBB3-12563 -  https://tracker.phpbb.com/browse/PHPBB3-12563
```
$message .= '<br /><br />' . $this->user->lang('STYLE_INSTALLED_RETURN_INSTALLED_STYLES', $this->u_base_action . '&mode=style');
```