Fix incorrect DNS record value

[aspearman]

What's being changed & why

This is a tiny change (one character!) to fix a typo in a DNS value in the docs, which could trip up users who are setting up their DNS records for GitHub pages. This is formatted to be copy-pasted and the extra "." may not be noticed. I haven't created a ticket because this is so minor; please let me know if I should create one anyway and I will. Thanks!

Check off the following

• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline (this link will be available after opening the PR).

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

[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

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
pages/configuring-a-custom-domain-for-your-github-pages-site/managing-a-custom-domain-for-your-github-pages-site.md	fpt ghec	fpt ghec

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

[nguyenalex836]

@aspearman Thanks so much for opening a PR with this fix! I'll update the branch and get this merged once tests are passing 🍏

[github-actions]

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