docs: Update decimal description

[sundy-li]

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Summary

Summary about this PR

Closes #issue

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databend	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Feb 27, 2023 at 9:00AM (UTC)

[mergify]

This pull request's title is not fulfill the requirements. @sundy-li please update it 🙏.

Valid format:

fix(query): fix group by string bug
  ^         ^---------------------^
  |         |
  |         +-> Summary in present tense.
  |
  +-------> Type: rfc, feat, fix, refactor, ci, docs, chore

Valid types:

• rfc: this PR proposes a new RFC
• feat: this PR introduces a new feature to the codebase
• fix: this PR patches a bug in codebase
• refactor: this PR changes the code base without new features or bugfix
• ci: this PR changes build/testing/ci steps
• docs: this PR changes the documents or websites
• chore: this PR only has small changes that no need to record