Fix upload action to account for markdown files

[Thenlie]

What does this PR do?

Improve repo

For resources

Description

I thought this check would only look at workflow files and thus use the .yml extension exclusively. This was not the case so this PR updates to also look for the .md extension.

Why is this valuable (or not)?

How do we know it's really free?

For book lists, is it a book? For course lists, is it a course? etc.

Checklist:

• [x] Read our contributing guidelines.
• [x] Search for duplicates.
• [x] Include author(s) and platform where appropriate.
• [x] Put lists in alphabetical order, correct spacing.
• [x] Add needed indications (PDF, access notes, under construction).
• [x] Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!