Clean up the example of sql to metricflow

[nhandel]

Making everything consistent in the example. It referenced returning customers but had a new customers = true.

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 or removing pages (delete if not applicable):

• [ ] Add/remove page in website/sidebars.js
• [ ] Provide a unique filename for new pages
• [ ] Add an entry for deleted pages in website/vercel.json
• [ ] Run link testing locally with npm run build to update the links that point to deleted pages

[welcome[bot]]

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. 😄

[vercel[bot]]

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