Fix wording to reflect poor performance of initial query before optimization

[aquarapid]

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.

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

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For details, see tips for choosing the affected versions.

• [x] master (the latest development version)
• [ ] v9.0 (TiDB 9.0 versions)
• [ ] v8.5 (TiDB 8.5 versions)
• [ ] v8.4 (TiDB 8.4 versions)
• [ ] v8.3 (TiDB 8.3 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)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)

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

see diff

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

[ti-chi-bot[bot]]

Hi @aquarapid. Thanks for your PR.

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[qiancai]

/ok-to-test

[qiancai]

/approve

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #21267.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #21268.