Document behavior of `prefix` in dependabot.yml

[schra]

Why:

It would have been helpful to me if the documentation was more straight-forward about that a : is appended to the commit title by default and how to disable that the : is appended (for example, by having trailing whitespace)

For reference, here is the implementation: https://github.com/dependabot/dependabot-core/blob/06702c83e5d964173504ddba6838c37fe37207ba/common/lib/dependabot/pull_request_creator/pr_name_prefixer.rb#L75-L83

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

Add an example, how to disable that a : is appended.

Check off the following:

• [x] 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).
• [x] For content changes, I have completed the self-review checklist.

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[github-actions[bot]]

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Content directory changes

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Source	Preview	Production	What Changed
code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file.md	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3

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

[cmwilson21]

@schra Thanks so much for opening a PR! I'll get this triaged for review :zap:

[github-actions[bot]]

Thanks for opening a pull request! We've triaged this issue for technical review by a subject matter expert :eyes:

[mchammer01]

Hi @schra - thanks for raising this issue ✨ I am going to liaise with the Dependabot Updates team and will come back to you once I've heard from them. Thanks for your patience 💖

[github-actions[bot]]

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 :zap:

[schra]

Awesome, thanks for merging and your input @mchammer01 :)