Update example for unique data test with concatenation of column names

[dbeatty10]

resolves #994

Problem

Two issues:

• The relevant code example was removed in Update data-tests.md docs.getdbt.com#6961
• The hyperlink is stale anyways

Solution

Replace the hyperlink with the explicit example.

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
• 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)