Fix documentation about mapping container port to a random free host port

[mindtraveller]

Why:

I found a mistake in the docs about port mapping in the services section of the Actions documentation. This PR corrects it.

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

The documentation page is changed.

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]

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[github-actions]

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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

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Source	Preview	Production	What Changed
actions/using-containerized-services/about-service-containers.md	fpt ghec ghes@ 3.5 3.4 3.3 3.2 3.1 ghae	fpt ghec ghes@ 3.5 3.4 3.3 3.2 3.1 ghae

[cmwilson21]

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