Update bigquery-setup.md

[jeremyyeo]

What are you changing in this pull request and why?

Re-explain what this config does in reality:

https://dbt-labs.slack.com/archives/C05FWBP9X1U/p1693003813907289?thread_ts=1692914554.692899&cid=C05FWBP9X1U

Checklist

• Review the Content style guide and About versioning so my content adheres to these guidelines.
• 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
• Ran link testing to update the links that point to the deleted page

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[mirnawong1]

🚢 @jeremyyeo !