Add missing download-dir-url entry

For now this entry is not used but it will be used in future versions of
slapcache. So make sure that from now on, the entry exists.

slapcache/template/update.cfg.in

@@ -5,6 +5,7 @@ key = %(key)s
 download-binary-cache-url = http://shacache.nxdcdn.com
 download-cache-url = https://shacache.nxdcdn.com
 download-binary-dir-url = http://shadir.nxdcdn.com
+download-dir-url = http://shadir.nxdcdn.com
 
 # Standard Signature handled by SlapOS Administrators
 signature-certificate-list =