[DOC BLD FIX] Remove todo:: directive from volume_driver
There was a '.. todo::' directive in cinder.interface.volume_driver
which is not a legitimate directive.  This was causing sphinx to
throw a warning.
This patch changes the format of the todo to avoid the warning.
@@ -148,7 +148,7 @@ class VolumeDriverCore(base.CinderInterface):
It should not export a LUN or actually make this storage available for
use, this is done in a later call.
- .. todo:: Add example data structure of volume object (smcginnis).
+ TODO(smcginnis): Add example data structure of volume object.
:param volume: Volume object containing specifics to create.
:returns: (Optional) dict of database updates for the new volume.
