Merge r1815480 from trunk:

address comment about substitution syntax

http://httpd.apache.org/docs/trunk/mod/mod_substitute.html#comment_7147




git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1815482 13f79535-47bb-0310-9956-ffa450edef68