gh-100174: [Enum] Correct PowersOfThree example. (GH-100178)

Changed from multiples of 3 to powers of 3 to match the class name.
(cherry picked from commit 868bab0fdc514cfa70ce97e484a689aee8cb5a36)

Co-authored-by: Beweeted <Beweeted@users.noreply.github.com>