Update instructions on documenting a singular test (#6363)

## What are you changing in this pull request and why?
Prompted from [this
thread](https://dbt-labs.slack.com/archives/C012TTNN0JK/p1729143266268959),
the instructions on documenting a test talk about _creating_ a new test
in yaml which isn't right

---------

Co-authored-by: Leona B. Campbell <[email protected]>

website/docs/docs/build/data-tests.md

@@ -68,7 +68,7 @@ having total_amount < 0
 
 The name of this test is the name of the file: `assert_total_payment_amount_is_positive`.
 
-To add a data test to your project, add a `.yml` file to your `tests` directory, for example, `tests/schema.yml` with the following content:
+To add a description to a singular test in your project, add a `.yml` file to your `tests` directory, for example, `tests/schema.yml` with the following content:
 
 <File name='tests/schema.yml'>