performance-tuning-practices: fix Scenario 2 CPU utilization value

[yahonda]

What is changed, added or deleted? (Required)

Fixes a data inconsistency in the Summary section: comparing Scenario 5 with Scenario 2, the bullet stated the average TiDB CPU utilization "drops from 936% to 577%," but 936% is actually Scenario 3's value (per line ~174: "The TiDB average CPU utilization increases from 874% to 936%"), not Scenario 2's. Scenario 2's actual value is 874% (per line ~117: "the average TiDB CPU utilization drops from 925% to 874%"). This looks like a copy-paste error from the adjacent bullet ("Comparing Scenario 7 with Scenario 3 ... drops from 936% to 478%"), which correctly uses 936% since it's comparing against Scenario 3.

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

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

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

• Related code change PR links (if applicable):
• This PR is translated from:
• Other reference link(s):

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

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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• performance-tuning-practices.md

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