centos image example was wrong

The following page had an incorrect
virt-install example for creating a
centos image:

http://docs.openstack.org/image-guide/content/centos-image.html

I fixed the example as described in the
associated bug, 1350564.

Change-Id: Ib687f8d46897f2375e2994bfedad286a8e0a8a12
Closes-Bug: #1350564
Merge "centos image example was wrong"