Fix incorrect DNS instructions

[vladh]

Why:

The previous instructions incorrectly implied that users should choose one of A, AAAA and ALIAS/ANAME records. In fact, if users do not opt for the ALIAS/ANAME record, they should use both A and AAAA records. This change makes that clearer.

(There is no issue for this change. I decided not to open one, because this is a minor change.)

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

(This is trivially visible in the diff.)

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on the review server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
pages/configuring-a-custom-domain-for-your-github-pages-site/managing-a-custom-domain-for-your-github-pages-site.md	fpt ghec	fpt ghec

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

🤖 This comment is automatically generated.

[Sharra-writes]

Thanks for opening a PR! I'll get this triaged for review.

[github-actions]

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