Update arenas.md
Update arenas.md

fixed typo
javatlacati/codecombat,javatlacati/codecombat,javatlacati/codecombat,codecombat/codecombat,codecombat/codecombat,jeremiahyan/codecombat,jeremiahyan/codecombat,codecombat/codecombat,codecombat/codecombat,codecombat/codecombat,jeremiahyan/codecombat,jeremiahyan/codecombat,javatlacati/codecombat,javatlacati/codecombat