BUG: Fix broken markdown on registration page

This commit fixes the following errors:

 PHP Warning:  DOMDocument::loadXML(): Opening and ending tag mismatch: br
   line 1 and error in Entity, line: 2 in /var/www/cdash/cdash/common.php on line 42,
   referer: http://localhost/cdash/register.php
 PHP Warning:  DOMDocument::loadXML(): Opening and ending tag mismatch: error
   line 1 and cdash in Entity, line: 2 in /var/www/cdash/cdash/common.php on line 42,
   referer: http://localhost/cdash/register.php
 PHP Warning:  DOMDocument::loadXML(): Premature end of data in tag cdash
   line 1 in Entity, line: 2 in /var/www/cdash/cdash/common.php on line 42,
   referer: http://localhost/cdash/register.php