[doc] Update _i18n.py example to pass pep8
[doc] Update _i18n.py example to pass pep8

It would be nice if the _i18n.py example can be copy-and-paste
to myapp/_i18n.py without pep8 errors.

Change-Id: I9ff4174ba1cac56c8b20ba572757fb54baa1e9bb

openstack/oslo.i18n