Update readme sample in creating-a-javascript-action.md

[TrianguloY]

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

On doc page https://docs.github.com/en/actions/creating-actions/creating-a-javascript-action#creating-a-readme there is a markdown sample with a ready to copy button.

If you copy/paste that sample as a new file, the output is the following:

This PR slightly improves that sample with the following two changes:

• The input and output are now h3 instead of h2, to differentiate from the input/output headers.
• The yaml sample is now inside triple-backtick for yaml code.

If you copy/paste this modification, the output is as follows:

Side-by-side screenshots

Check off the following:

• I have reviewed my changes in staging (look for the "Automatically generated comment" and click the links in the "Preview" column to view your latest changes).
• For content changes, I have completed the self-review checklist.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[github-actions]

Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
actions/creating-actions/creating-a-javascript-action.md	fpt ghec ghes@ 3.6 3.5 3.4 3.3 3.2 ghae	fpt ghec ghes@ 3.6 3.5 3.4 3.3 3.2 ghae

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

[cmwilson21]

@TrianguloY Thanks so much for opening a PR! I'll get this triaged for review ⚡

[lucascosti]

Thanks for this, @TrianguloY! I'll get it merged 🚀

[github-actions]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours. If you're looking for your next contribution, check out our help wanted issues ⚡