Remove referances to LegacyFormatter in example logging.conf.

A recent change (Ibe41afc6e9d7b432453785819821b3c8f0078613), removed
nova.openstack.common.log.LegacyFormatter, and replaced it with
ContextFormatter.

This change updates the example logging config file to reflect this
change, as any reference to the now-removed LegacyFormatter causes
nova services to fail to start.

Change-Id: I55a984ed7ea12c1dde49c4eb13d5b4f47471bf95