This is a follow up to patch Ia3fd947ff619c11ff0ce474897533dcf7b49d9b3.
One of the imports for the keystone session example was wrong. It was
trying to import v3 like:
from keystoneauth1 import v3
Whereas in my testing v3 actually exists deeper under identity:
from keystoneauth1.identity import v3
This patch fixes this import in the example documentation.
Change-Id: Ie31144fe65116d20a36b83a96079a3804bf55f29