Fix regression in artifact-attestations-step-for-container-images.md

[snorremd]

Why:

Fixes regression where the reusable artifact attestations step example was accidentally broken by a commit that replaced dashes with asterisks.

Commit that broke example: 0886a39

What's being changed (if available, include any code snippets, screenshots, or gifs):

Swapped an asterisk in a GitHub actions step example with a dash.

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Content directory changes

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Source	Preview	Production	What Changed
actions/publishing-packages/publishing-docker-images.md	fpt ghec ghes@ 3.13 3.12 3.11 3.10 3.9	fpt ghec ghes@ 3.13 3.12 3.11 3.10 3.9	from reusable

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[snorremd]

This seems to be a bit more complicated to solve than I initially thought.

1. The attestation step reusable file seems to be the only file that contains only YML.
2. The markdown linter configured for the project will throw a fit if you use a dash -
3. The templating system does not appear to recognise other extensions than md, so renaming the file to use the yml extension is no good

Not really sure how to solve this, so closing my PR and will add an issue instead so that the core contributors can fix it.