Fix angle in hints

Summary: For https://khanacademy.atlassian.net/browse/AI-401846

Test Plan: Make sure hints for exercises/inscribed_angles_1.html?seed=173&problem=find-central-3&debug&lang=en refer to the correct angles

Reviewers: eater, emily

Reviewed By: emily

Differential Revision: https://phabricator.khanacademy.org/D15007