correct GlobalID mixin name in the guides

This needs to be `GlobalID::Identification`. In version 4.2.0.beta2, `ActiveModel` does not have a `GlobalIdentification` namespace. I was using `ActiveJob` with a `Mongoid` model when I came across this issue. Mixing in `GlobalID::Identification` fixed it for me. So I think an update to the guide is all that is necessary.