Update cs3.md
Update cs3.md

updated arenas lesson to reflect changes made across all arenas lesson plans - reframing as capstone challenge and bringing language and organization in line with teacher expectations.
codecombat/codecombat,javatlacati/codecombat,jeremiahyan/codecombat,javatlacati/codecombat,javatlacati/codecombat,codecombat/codecombat,jeremiahyan/codecombat,javatlacati/codecombat,jeremiahyan/codecombat,javatlacati/codecombat,jeremiahyan/codecombat,codecombat/codecombat,codecombat/codecombat,codecombat/codecombat