Update start.rst to reference specific DAG

[joe-gordian-software]

Update start.rst to reference specific DAG to reference example_bash_operator specifically, rather than "the example dag" because there are multiple example dags.

Issue link: WILL BE INSERTED BY boring-cyborg

• Description above provides context of the change
• Commit message/PR title starts with [AIRFLOW-NNNN]. AIRFLOW-NNNN = JIRA ID^*
• Unit tests coverage for changes (not needed for documentation changes)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

^* For document-only changes commit message can start with [AIRFLOW-XXXX].

---

In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community!
If you have any issues or are unsure about any anything please check our
Contribution Guide (https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst)

Here are some useful points:

• Pay attention to the quality of your code (flake8, pylint and type annotations). Our pre-commits
  will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory).
  Adding a new operator? Consider adding an example DAG that shows how users should use it.
• Consider using Breeze environment for testing
  locally, it’s a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from
  Committers.

Apache Airflow is a community-driven project and together we are making it better 🚀.

In case of doubts contact the developers at:
Mailing List: dev@airflow.apache.org
Slack: https://apache-airflow-slack.herokuapp.com/

[boring-cyborg]

Awesome work, congrats on your first merged pull request!