Fix consumer create interface documentation

The following discussion from !14425 should be addressed:

• @qzhaogitlab started a discussion: (+1 comment)

  minor: seems the recent commits changed the method name to find_or_create

  1. Run `Billing::Usage::Consumers.find_or_create`:
     1. `.com`: `Billing::Usage::Consumers.find_or_create(entity_id: <gitlab_user_id here>, source: subscription, gl_namespace_id: <group_id here>).execute`
     1. `Self-Managed`: `Billing::Usage::Consumers.find_or_create(entity_id: <gitlab_user_id here>, source: subscription, self_managed_instance_activation: most_recent_activation).execute`