Fix wrong instruction to reference a workflow

[benoitchantre]

Why:

The instructions to reference a workflow mention branch and tags instead of SHA to securely reference a workflow. This doesn’t follow the best practices.

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

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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

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/using-workflows/reusing-workflows.md	fpt ghec ghes@ 3.6 3.5 3.4	fpt ghec ghes@ 3.6 3.5 3.4

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

[cmwilson21]

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

[lucascosti]

Thanks a lot for this fix, @benoitchantre!

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