[Docs] Passing the `unique` data test a concatenation of column names

[dbeatty10]

Describe the docs update

This example will need updating after dbt-labs/docs.getdbt.com#6961.

passing the unique test a concatenation of the columns (as discussed here).

Two things in particular:

1. The link is already out-of-date
2. Even if we update the link, it will no longer include the relevant content after Update data-tests.md docs.getdbt.com#6961 is merged

So we should either:

1. Remove that line entirely, or
2. Add an example like the following:

       tests:
         - unique:
             column_name: "country_code || '-' || order_id"