Fix wording to reflect poor performance of initial query before optimization

[qiancai]

What is changed, added or deleted? (Required)

Fix wording in the explain walkthrough, the current working seems to imply that the initial performance is good (which it is not). The rest of the walkthrough is to improve the performance, so the changed wording seems more correct.

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

This PR is translated from: pingcap/docs#20840

Do your changes match any of the following descriptions?

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

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[qiancai]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #20572.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #20573.