Docs: add reference to the dbt-core jaffle shop project (#583)

A quick fix to add a hyperlink that references the jaffle-shop project by
dbt-labs

README.rst

@@ -37,7 +37,7 @@ Check out the Quickstart guide on our `docs <https://astronomer.github.io/astron
 Example Usage
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-You can render an Airflow Task Group using the ``DbtTaskGroup`` class. Here's an example with the jaffle_shop project:
+You can render an Airflow Task Group using the ``DbtTaskGroup`` class. Here's an example with the `jaffle_shop project <https://github.com/dbt-labs/jaffle_shop>`_:
 
 
 .. code-block:: python