Replace github with git.openstack.org

The git.openstack.org has been the recommended git repo for OpenStack
projects.

Change-Id: Ibc973efec60a5f1c9c7a584460b4c00e2d4f7b2e

README.rst

@@ -25,7 +25,7 @@ The following should get you started::
 
 Clone the repository into your local OpenStack directory:
 
-    $ git clone https://github.com/openstack/mistral-dashboard.git
+    $ git clone https://git.openstack.org/openstack/mistral-dashboard.git
 
 Install mistral-dashboard