Fix warning while building docs
Fix warning while building docs

Seen locally:

    Warning, treated as error:
    .../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.replicator.get_data_dir,
    swift.obj.diskfile.get_data_dir

Not sure why it hasn't been seen in the gate...

The whole sentence is suspect, though; the Diskfile class doesn't define
a get_data_dir method, though it uses the module-level get_data_dir...

Change-Id: I6855c82315e1c71596ecce25b66b54133c239377

openstack/swift,openstack/swift,openstack/swift,openstack/swift,swiftstack/swift,tipabu/swift,swiftstack/swift,swiftstack/swift,tipabu/swift,tipabu/swift,tipabu/swift