In the dev-ref, the hint key for volume create API
should be 'OS-SCH-HNT:scheduler_hints' but not 'scheduler_hints'.
Furthermore, it should be parallel with the key 'volume'.
To be more clear fix the example as well.
Change-Id: I999aebb0e53ebac5dc121ef1ccdc98f9d5ac816a
f63d89dd3b