Replace hyperlink for using a SQL expression in a data test

[dbeatty10]

Problem

While writing up dbt-labs/docs.getdbt.com#6436, noticed that the content of a link got moved to a different page in the dbt product documentation.

Solution

Replace with the link to the new page + section heading.

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• N/A I have run this code in development and it appears to resolve the stated issue
• Tests are not required/relevant for this PR
• I have updated the README.md (if applicable)