fix frenchism s/exemple/example/

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
example: use rbd pool instead of data

Because it exists by default and data no longer does.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #1 from dachary/wip-example

minor fixes to the example