Fix outdated statement about TiDB's default transaction mode in optimistic-transaction.md (#21921) · pingcap/docs@bf0ff7e

[vale] reported by reviewdog 🐶 [PingCAP.EnDash] Use an em dash ('—') instead of en dash ('–'). Raw Output: {"message": "[PingCAP.EnDash] Use an em dash ('—') instead of en dash ('–').", "location": {"path": "optimistic-transaction.md", "range": {"start": {"line": 71, "column": 94}}}, "severity": "ERROR"}