disabled mailman staging.txt because it failed in a buildbot run

This is the log of the failure

Failure in test lib/canonical/launchpad/mailman/tests/../doc/staging.txt
Failed doctest test for staging.txt
  File "lib/canonical/launchpad/mailman/tests/../doc/staging.txt", line 0

----------------------------------------------------------------------
File "lib/canonical/launchpad/mailman/tests/../doc/staging.txt", line 169, in staging.txt
Failed example:
    sync.dump_list_info()
Differences (ndiff with -expected +actual):
      fake-team
          lists.launchpad.dev http://lists.launchpad.dev/mailman/
          No Launchpad team: fake-team
      staging-one
          lists.prod.launchpad.dev http://lists.prod.launchpad.dev/
          <email address hidden>
      staging-two
    - lists.prod.launchpad.dev http://lists.prod.launchpad.dev/
    ? ----- -----
    + lists.launchpad.dev http://lists.launchpad.dev/mailman/
    ? ++++++++
          <email address hidden>
          <email address hidden>