Update where.md to enhance the usability of the example

[brian-franklin]

The example on the page is useful; however, it is rigid in only allowing a 3 day look back. Adjusting the pattern to include a number value and a macro that performs a regex search to set the dateadd logic to whatever value is configured in the test. This allows for reusbaility of the macro for different lookback days.

What are you changing in this pull request and why?

Checklist

• I have reviewed the Content style guide so my content adheres to these guidelines.
• The topic I'm writing about is for specific dbt version(s) and I have versioned it according to the version a whole page and/or version a block of content guidelines.
• I have added checklist item(s) to this list for anything anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."
• The content in this PR requires a dbt release note, so I added one to the release notes 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.
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