docs: fix single upgrade markdown link

[froblesmartin]

Why:

Closes: n/a

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

It seems the title changed from "single" to "standalone", but was missed in a couple of links.

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• 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
admin/enterprise-management/updating-the-virtual-machine-and-physical-resources/upgrading-github-enterprise-server.md	ghes@ 3.8 3.7 3.6 3.5 3.4	ghes@ 3.8 3.7 3.6 3.5 3.4

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

[steveward]

Thanks for fixing these links @froblesmartin, I'll get this merged!

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