Fix a typo in tidb-monitoring-api.md (#6829)

[ti-chi-bot]

This is an automated cherry-pick of #6829

What is changed, added or deleted? (Required)

Fix a typo in tidb-monitoring-api.md

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• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

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[ti-chi-bot]

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[TomShawn]

This version does not need cherry-pick.