It sounds like you might have made some type of syntax error whilst editing. I made this same change on my site a while back so this is what I did:
Make a copy of includes\languages\english\email_extras.php
and put it into your custom template.
Open up the copy and at around line 33, I changed..
PHP Code:
define('EMAIL_ADVISORY', '-----' . "\n" . '<strong>IMPORTANT:</strong> For your protection and to prevent malicious use, all emails sent via this web site are logged and the contents recorded and available to the store owner. If you feel that you have received this email in error, please send an email to ' . STORE_OWNER_EMAIL_ADDRESS . "\n\n");
to..
PHP Code:
define('EMAIL_ADVISORY', '-----' . "\n" . '<strong>IMPORTANT:</strong> For your protection and to prevent malicious use, all emails sent via this web site are logged and the contents recorded and available to the store owner. If you feel that you have received this email in error, please visit our website and contact us using our contact form.');