Whenever you manually make changes on a server running plesk that will effect your httpd.conf Apache configuration file you have to tell plesk to rebuild the httpd.conf file. Luckily it is very easy to tell plesk to rebuild httpd.conf
Simply login to SSH as a user with sudo privileges and type one of the commands below to re-build the Apache configuration file httpd.conf
sudo /usr/local/psa/admin/sbin/websrvmng -v -a
sudo /usr/local/psa/admin/bin/websrvmng -v -a
You can try both commands without worrying about it messing anything up. The websrvmng file could be in sbin or bin it all just depends on how plesk was installed on your server.