admin show ddl: Fix EBNF

[dveeden]

What is changed, added or deleted? (Required)

The EBNF didn't correctly indicate that ADMIN SHOW DDL is a valid statement

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Do your changes match any of the following descriptions?

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-09 08:59:14.633639745 +0000 UTC m=+348051.868873858: ☑️ agreed by Oreoxmt.
• 2024-07-09 09:04:29.386328398 +0000 UTC m=+348366.621562511: ☑️ agreed by qiancai.

[dveeden]

/hold

[dveeden]

/unhold

[dveeden]

https://github.com/pingcap/tidb/blob/d8098b970893c850cc318727d9dc336d0a677f35/pkg/executor/executor.go#L361-L371

Here you can see that there are multiple queries that are put into the QUERY field.

[Oreoxmt]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Oreoxmt

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #18254.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #18255.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #18256.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #18257.