Fix doc as spotted by Mike Matthews in online doc

Fix link to distcache. 
http://www.distcache.org/ --> http://distcache.sourceforge.net/

(r1740717 in trunk)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1740719 13f79535-47bb-0310-9956-ffa450edef68