fix(docs): correct explanation of sum expression for order_total

[atcol]

What are you changing in this pull request and why?

Checklist

• Review the Content style guide so my content adheres to these guidelines.
• For docs versioning, review how to version a whole page and version a block of content.
• Add a checklist item for anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

Adding new pages (delete if not applicable):

• Add page to website/sidebars.js
• Provide a unique filename for the new page

Removing or renaming existing pages (delete if not applicable):

• Remove page from website/sidebars.js
• Add an entry website/static/_redirects
• Run link testing locally with npm run build to update the links that point to the deleted page

[welcome]

Hello!👋 Thanks for contributing to the dbt product documentation and opening this pull request! ✨
We use Markdown and some HTML to write the dbt product documentation. When writing content, you can use our style guide and content types to understand our writing standards and how we organize information in the dbt product docs.
We'll review your contribution and respond as soon as we can. 😄

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Name	Status	Preview	Comments	Updated (UTC)
docs-getdbt-com	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 22, 2023 11:05am

[mirnawong1]

hey @atcol ! thanks for opening this pr up and fixing the typo! happy to get this merged as it's much clearer for everyone - looking forward to future contributions ✨