You used the file editor - this is a known bug, which drops the \ mark when you edit things. You should really use a real FTP program for doing any editing, especially in those language files where the \ is everywhere!
Here's the original code - save it and upload it with FTP. I even removed the 2-5 days for you :)
<?php
/*
$Id: checkout_success.php,v 1.12 2003/04/15 17:47:42 dgw_ Exp $
osCommerce, Open Source E-Commerce Solutions
http://www.oscommerce.com
Copyright (c) 2002 osCommerce
Released under the GNU General Public License
*/
define('NAVBAR_TITLE_1', 'Checkout');
define('NAVBAR_TITLE_2', 'Success');
define('HEADING_TITLE', 'Your Order Has Been Processed!');
define('TEXT_SUCCESS', 'Your order has been successfully processed!');
define('TEXT_NOTIFY_PRODUCTS', 'Please notify me of updates to the products I have selected below:');
define('TEXT_SEE_ORDERS', 'You can view your order history by going to the <a href="' . tep_href_link(FILENAME_ACCOUNT, '', 'SSL') . '">\'My Account\'</a> page and by clicking on <a href="' . tep_href_link(FILENAME_ACCOUNT_HISTORY, '', 'SSL') . '">\'History\'</a>.');
define('TEXT_CONTACT_STORE_OWNER', 'Please direct any questions you have to the <a href="' . tep_href_link(FILENAME_CONTACT_US) . '">store owner</a>.');
define('TEXT_THANKS_FOR_SHOPPING', 'Thanks for shopping with us online!');
define('TABLE_HEADING_COMMENTS', 'Enter a comment for the order processed');
define('TABLE_HEADING_DOWNLOAD_DATE', 'Expiry date: ');
define('TABLE_HEADING_DOWNLOAD_COUNT', ' downloads remaining');
define('HEADING_DOWNLOAD', 'Download your products here:');
define('FOOTER_DOWNLOAD', 'You can also download your products at a later time at \'%s\'');
?>