Fix command typo in managing_daemons.diviner

Summary:
Fix the 'managing daemons' doc's example for launching daemons in a
way suitable for multiple web servers. Separate the '--no-discovery'
argument with '--', otherwise it doesn't appear to be understood.

Test Plan:
Attempt to launch daemon in various ways, make sure to include
ways that are expected to fail, otherwise it may not be doing what
we expect.

phd launch PhabricatorRepositoryPullLocalDaemon --no-discovery
.. errors in daemon log 'unrecognised argument' ..
phd launch PhabricatorRepositoryPullLocalDaemon -- --not-an-arg
.. errors in daemon log 'unrecognised argument' ..
phd launch PhabricatorRepositoryPullLocalDaemon -- --no-discovery
.. no errors in daemon log ..

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9374