explain joins: fix the TableReader operator

[Oreoxmt]

What is changed, added or deleted? (Required)

Keep the description of TableReader ID to be consistent with the output.

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v7.4 (TiDB 7.4 versions)
• v7.3 (TiDB 7.3 versions)
• v7.2 (TiDB 7.2 versions)
• v7.1 (TiDB 7.1 versions)
• v7.0 (TiDB 7.0 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

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

• This PR is translated from: explain joins: fix the TableReader operator docs-cn#15103
• Other reference link(s): *: update the row count display of probe side of IndexJoin and Apply docs-cn#11624

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]

[LGTM Timeline notifier]

Timeline:

• 2023-09-27 03:15:46.04253947 +0000 UTC m=+417535.760881688: ☑️ agreed by qiancai.

[ti-chi-bot]

@time-and-fate: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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[Oreoxmt]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #14935.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #14936.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.2: #14937.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.3: #14938.

[ti-chi-bot]

In response to a cherrypick label: cannot checkout release-7.4: error checking out release-7.4: exit status 1. output: error: pathspec 'release-7.4' did not match any file(s) known to git