fix pd cert allowed cn description

[lhy1024]

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What is changed, added or deleted? (Required)

fix pd cert allowed cn description, according tikv/pd#5134 and https://docs.pingcap.com/zh/tidb-in-kubernetes/stable/enable-tls-between-components#%E7%AC%AC%E4%BA%8C%E6%AD%A5%E9%83%A8%E7%BD%B2-tidb-%E9%9B%86%E7%BE%A4

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-30 01:48:30.478669484 +0000 UTC m=+321864.235805050: ☑️ agreed by hfxsd.

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[hfxsd]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #17263.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.0: #17264.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #17265.