A directory needs to have a trailing slash. A link that does not have the trailing slash is looked at as being a file first and when no file is found the web server redirects to the "Server Name" of the web site with the URL modified to then have a trailing slash.
To prevent the web server from changing the URL to the "Server Name" insert the following piece of code into a file named .htaccess.
RewriteEngine on
RewriteBase /
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^(.+[^/])$ $1/ [R]