Geo: `model` on replicator should be class method

What does this MR do?

Another small fix in the Geo Self-service Framework documentation.

The method model on the replicator should be a class method. And generally the class methods are defined before the instance methods.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content