AIRFLOW-XXXX Update docs on example files for k8s

[frankcash]

Update docs on example files for k8s as there is no longer an airflow.yaml file in scripts/ci/kubernetes/app/, but there is now a secrets.yaml

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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].

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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.

[potiuk]

Thanks @frankcash !

[potiuk]

Merged! Just next time pls. add [AIRFLOW-XXXX] in commit (there was no []). I added them while merging so no problem this time.

[frankcash]

Will do!