=clu #3536 Update doc of hostname port params in cluster jmx script

* This was changed in the switch to jmxsh (72aa5ef), but since
  we released with space separator I updated the documentation to that
  format.
(cherry picked from commit a731f3b6b6240ff77e705269538c37f869949ff0)
Merge pull request #1659 from akka/wip-3536-cluster-jmx-script-2.2-patriknw

=clu #3536 Update doc of hostname port params in cluster jmx script (backport)