Docs: incorrect name for storage config option in README.rst

[Md Nadeem]

"message_storage" should replace with "message_store" in [drivers] section of README.rst

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/223906

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/223906
Committed: https://git.openstack.org/cgit/openstack/zaqar/commit/?id=02b6c93719a0ae3655f460c74e6d2037567c3a35
Submitter: Jenkins
Branch: master

commit 02b6c93719a0ae3655f460c74e6d2037567c3a35
Author: MD NADEEM <email address hidden>
Date: Wed Sep 16 12:15:45 2015 +0900

    Fix "message_store" config option in zaqar doc

    In zaqar documents, a config option under the [driver] section is wrongly named as "message_storage".
    However actually as per zaqar code it should be name as "message_store".
    This patch fix this name issue.
    Closes-Bug: #1496202

    Change-Id: I7fdff8b7cb87850920f7144aea84e907221bbcf7