docs: use %H instead of a hardcoded IP

In https://coreos.com/docs/launching-containers/launching/launching-containers-fleet/
Run a Simple Sidekick, it uses %H.  Not sure why it would be using a
hardcoded ip rather than %H.
Merge pull request #1006 from jonboulle/tleyden-patch-1

docs: use %H instead of a hardcoded IP