alert rules: fix wrong coprocessor alert rules

[en-jin19]

What is changed, added or deleted? (Required)

fixed wrong coprocessor alert rules in "altert rules.md".

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• master (the latest development version)
• 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)

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

• This PR is translated from: fix wrong coprocessor alert rules docs-cn#6904
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[TomShawn]

This PR is replaced by #6250.