mod_brotli: Properly describe the "no-brotli" environment variable in

the documentation.

The previous description explained the semantics of this variable as
if it has been a (non-existing) "force-brotli".


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1790868 13f79535-47bb-0310-9956-ffa450edef68