Job consumption example in Job documentation has incorrect usage of persistence backend

[Min Pae]

Job consumption example in Job documentation calls destroy_logbook on the persistence backend instance when the method lives in the connection, not the backend.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/145574
Committed: https://git.openstack.org/cgit/openstack/taskflow/commit/?id=a18a939aae1dff42a4741a1852829abe5280aa80
Submitter: Jenkins
Branch: master

commit a18a939aae1dff42a4741a1852829abe5280aa80
Author: Min Pae <email address hidden>
Date: Wed Jan 7 12:26:19 2015 -0800

    Fix for job consumption example using wrong object

    Change job consumption example in Job documentation to use
    persistence.get_connection().destroy_logbook() instead of
    persistence.destroy_logbook()

    Change-Id: Ia6d0f2be4dd7cd161ef7b6d8afc32aab532fd69b
    Closes-Bug: 1408434