Merge r1723522 from trunk:

* mod_log_debug docs: Fix the IS_SUBREQ example

Submitted by: jkaluza
Reviewed by: jorton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1823289 13f79535-47bb-0310-9956-ffa450edef68