The concept of a "Release Sprint" is not found in the Scrum Guide or the Professional Scrum Product Owner™ (PSPO I) materials. Scrum does not require a separate Sprint for releasing an Increment. According to Scrum, each Increment must be usable and potentially releasable at the end of every Sprint, which means the product should be in a releasable state irrespective of whether the Product Owner decides to release it or not. The idea of a Release Sprint is not aligned with Scrum's principle of continuous delivery and the definition of "Done".
The Scrum Guide and the Professional Scrum Product Owner™ (PSPO I) study materials emphasize the importance of the Increment being releasable after every Sprint without the need for an additional Release Sprint12.