At the moment, the `get_data_dir` ref in [0] is not
pointing to anything. This patch amends that and
links it to the correct policy string.
Without updating this change, local tox builds
for swift documentation fail with the following
error: Warning, treated as error: /home/asettle/openstack/swift/doc
/source/overview_policies.rst:555:more than one target found for
cross-reference u'get_data_dir': swift.obj.reconstructor.get_data_dir,
swift.obj.diskfile.get_data_dir, swift.obj.replicator.get_data_dir
[0] https://docs.openstack.org/swift/rocky/overview_policies.html#object-server
Change-Id: I7c699e4fc46706a4971fce5a85ed335f471d3a2b
693f931a59