Update monitor-a-tidb-cluster.md

[lilyjazz]

First-time contributors' checklist

• [x] I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added, or deleted? (Required)

Which TiDB Operator version(s) do your changes apply to? (Required)

• [x] master (the latest development version)
• [x] v1.3 (TiDB Operator 1.3 versions)
• [ ] v1.2 (TiDB Operator 1.2 versions)
• [ ] v1.1 (TiDB Operator 1.1 versions)
• [ ] v1.0 (TiDB Operator 1.0 versions)

What is the related PR or file link(s)?

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[ti-chi-bot]

[REVIEW NOTIFICATION]

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• ran-huang

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