Fixing typo in AVAILABLE_REGIONS documentation

[Shaoquan Chen]

Currently it says: A tuple of tuples which define multiple regions. But actually it is a list of tuples, like:

AVAILABLE_REGIONS = [
    ('http://cluster1.example.com:5000/v2.0', 'cluster1'),
    ('http://cluster2.example.com:5000/v2.0', 'cluster2'),
]

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/172697
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=fc6ef2bd057e44a69b8f2cbb08e833f871d886cc
Submitter: Jenkins
Branch: master

commit fc6ef2bd057e44a69b8f2cbb08e833f871d886cc
Author: Shaoquan Chen <email address hidden>
Date: Sat Apr 11 22:32:34 2015 -0700

    Fixing typo in AVAILABLE_REGIONS section

    Currently it says: A tuple of tuples which define multiple regions. But
    actually it is a list of tuples, like:

    AVAILABLE_REGIONS = [
        ('http://cluster1.example.com:5000/v2.0', 'cluster1'),
        ('http://cluster2.example.com:5000/v2.0', 'cluster2'),
    ]

    Change-Id: Icbb4799689d650bac0bc17737b5747cff10a6154
    Closes-Bug: #1442965