Update README.md

[kthogiti]

Made a correction in the Upgrade documentation section

• [x] You've read the Contributor Guide and Code of Conduct.
• [x] You've included unit or integration tests for your change, where applicable.
• [x] You've included inline docs for your change, where applicable.
• [x] There's an open issue for the PR that you are making. If you'd like to propose a new feature or change, please open an issue to discuss the change or find an existing issue.

For PRs which target a specific extension within UA, changes won't be approved by a member of the dotnet-upgrade-assistant-admin group until a member of the code owners of the extension has approved, when required.

Description

Made typo correction change under "Upgrade Documentation"

Addresses #bugnumber (in this specific format)

[mgoertz-msft]

This PR no longer applies to this repo.