docs: fix create issue screen: rename config.yaml -> yml (#11816)

- the [create
issue](https://github.com/DataDog/dd-trace-py/issues/new/choose) screen
has issue templates but no external links
- the only difference I can see is that I used config.yaml whereas in
other repos it's config.yml
  - very confusing since the issue templates use .yaml...
- follow up to #11765

## Checklist
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## Reviewer Checklist
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[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
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[release branch maintenance
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