Fix the broken template of default_airflow

[CastielWong]

I tried to apply "dockerrize" to render the "default_airflow.cfg" template for my Airflow, yet Airflow failed to start with the new config file.

It spent me quite a while to figure out what had happened. When I double checked the "airflow.cfg" auto-generated by airflow db init, I finally realized the reason why it failed was because of those extra brackets for delims in the template.

So this PR is to fix the issue by removing those extra brackets in the template.

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[boring-cyborg]

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[github-actions]

The Workflow run is cancelling this PR. Building images for the PR has failed. Follow the the workflow link to check the reason.

[github-actions]

The Workflow run is cancelling this PR. Building images for the PR has failed. Follow the the workflow link to check the reason.

[github-actions]

The Workflow run is cancelling this PR. Building images for the PR has failed. Follow the the workflow link to check the reason.

[github-actions]

The Workflow run is cancelling this PR. It has some failed jobs matching ^Pylint$,^Static checks,^Build docs$,^Spell check docs$,^Backport packages$,^Provider packages,^Checks: Helm tests$,^Test OpenAPI*.

[CastielWong]

Looks like this issue is caused by "pylint", can't fix it without reconfig pre-commit thus close the PR.