switched another real domainname to example.org

* Changed the value of missing-files.plugins.semsg.broker.host to a better
  example hostname.

This should go to the release notes, as a previous value pointed to a real
server at CERN.

Require-notes: yes
Require-book: no

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>