Fit disable-selfhosted-runners reusable sentence in its ifversion

[janbrasna]

Why:

Fixes double fullstops by moving one inside its conditional.

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

https://docs.github.com/en/actions/security-for-github-actions/security-guides/security-hardening-for-github-actions#hardening-for-self-hosted-runners

                              ^^

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 a 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.

[github-actions]

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Content directory changes

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Source	Preview	Production	What Changed
actions/hosting-your-own-runners/managing-self-hosted-runners/about-self-hosted-runners.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	from reusable
actions/hosting-your-own-runners/managing-self-hosted-runners/adding-self-hosted-runners.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	from reusable
actions/hosting-your-own-runners/managing-self-hosted-runners/monitoring-and-troubleshooting-self-hosted-runners.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	from reusable
actions/security-for-github-actions/security-guides/security-hardening-for-github-actions.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	from reusable

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

[nguyenalex836]

@janbrasna Thank you so much for opening a PR with this fix! ✨ We are currently in a repo freeze, but we'll get this merged as soon as the freeze ends on Jan 6th 💛