Fix example goose migration path. (#3155)
Fix example goose migration path. (#3155)

The contrib guide to migrations creates a migration with goose create AddWizards sql but references it later as
sa/_db/20160915101011_WizardMigrations.sql. This commit fixes the
reference to use the correct AddWizards.sql filename suffix.
ibukanov/boulder,lucasallan/boulder,bmw/boulder,jsha/boulder,jfrazelle/boulder,letsencrypt/boulder,ibukanov/boulder,jsha/boulder,letsencrypt/boulder,jsha/boulder,bmw/boulder,bmw/boulder,jcjones/boulder,ibukanov/boulder,jfrazelle/boulder,lucasallan/boulder,jcjones/boulder,jfrazelle/boulder,jcjones/boulder,jcjones/boulder,letsencrypt/boulder,jfrazelle/boulder,lucasallan/boulder