Fix outdated statement about TiDB's default transaction mode in optimistic-transaction.md (#21921)#21924

ti-chi-bot[bot] merged 1 commit intopingcap:release-8.1pingcap/docs:release-8.1from

ti-chi-bot:cherry-pick-21921-to-release-8.1ti-chi-bot/docs:cherry-pick-21921-to-release-8.1Copy head branch name to clipboard