The following note in the Agile Accelerator getting started Guide is wrong. (page 9)

"Note: You can change the sprint naming convention by modifying the validation rule on the Sprint object in Setup. From the object management settings for sprints, go to Validation Rules, and then modify or de-activate the Verify Name Format validation rule."

We wont be able to modify the validation Rule, the only available option is to inactivate it and clone it to rewrite our own validation Rule.