planner: fix grouping sets document typo

[AilinKid]

What problem does this PR solve?

Issue Number: ref #42631

Problem Summary:

What changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[tiprow]

Hi @AilinKid. Thanks for your PR.

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[AilinKid]

/ok-to-test

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, qw4990

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-02 09:15:23.65151721 +0000 UTC m=+1315850.137006042: ☑️ agreed by hawkingrei.
• 2024-07-02 09:15:58.180438153 +0000 UTC m=+1315884.665926975: ☑️ agreed by qw4990.