Fix failing openstack-tox-docs job for image source path

The path is wrong and the openstack-tox-docs job fails blocking
the gate as seen at [1]

[1] https://d112d82adaf9d03da6e7-2f18d96c12ded5c9f86090319cfc832d.ssl.cf2.rackcdn.com/772442/17/check/openstack-tox-docs/925f008/job-output.txt

Change-Id: I1e2bba57911e15ca17e6416dafcaf62fe1ac1dcc

specs/rocky/split-controlplane.rst

@@ -133,7 +133,7 @@ respect to managing instances, there would be no reason that the
 workload itself could not perform adequately once running and each
 workload site would have a smaller footprint.
 
-.. image:: ../../images/split-controlplane/ceph-details.png
+.. image:: ../../../../images/split-controlplane/ceph-details.png
    :height: 445px
    :width: 629px
    :alt: Diagram of an example control site with multiple workload sites