update in authentication to follow the content style guide

[AdityaSrivastava185]

Why:

Periods need to be removed from each list line, as each line is not a complete sentence (per our style guide)

Closes: Remove periods from list in about-mandatory-two-factor-authentication#about-eligibility-for-mandatory-2fa #34521

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

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.

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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
authentication/securing-your-account-with-two-factor-authentication-2fa/about-mandatory-two-factor-authentication.md	fpt ghec	fpt ghec

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

[nguyenalex836]

@AdityaSrivastava185 Thank you so much for opening a PR! ✨ Unfortunately, the following PR has already been set to merge and address this issue once our repo freeze ends on 9/16 - #34524

With that said, if you're looking for another issue to pick up while waiting, take a look at our help wanted section to find open issues you can work on 💛

Thank you for your willingness to contribute to our community!