sql: fix ebnf diagram

[ran-huang]

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• v6.6 (TiDB 6.6 versions)
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What is the related PR or file link(s)?

• This PR is translated from: fix wrong diagram in create/alter user docs-cn#12574, sql: fix ebnf diagram docs-cn#12579
• Other reference link(s):

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]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• shichun-0415

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[shichun-0415]

LGTM

[ran-huang]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 90a1aa4

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #11938.