Prevent dag-processor crash on encountering Excel files in the DAG directory

[jstaf]

One of our data analysts found a novel way to crash Airflow's dag-processor by uploading an Excel file to Airflow's DAG directory! Normally, Airflow should print a warning message and say that the Excel file is an invalid zip file (MS Office .xlsx/.docx/etc. documents are all zip archives). In this case however, Airflow has the wrong number of %ss when logging the warning, which causes Airflow's dag-processor to crash when handling the zipfile.BadZipFile exception. This PR fixes the crash by fixing the warning message to have the correct number of %ss in its format string.

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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.

[github-actions]

Backport successfully created: v3-1-test

Status	Branch	Result
✅	v3-1-test