Update install-copilot-extension.md for neovim on Windows

[kimfaint]

replace line continuation character and home directory variable

Why:

Closes:

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

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 an 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 and the changes look good in the review environment.

[Copilot]

Pull Request Overview

This PR updates the Neovim and Vim installation instructions for Windows by replacing Unix-style line continuation and home directory syntax with Windows-appropriate equivalents.

• Replaces backslash (\) line continuation with caret (^) for Windows command line
• Replaces $HOME environment variable with %USERPROFILE% for Windows paths

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Source	Review	Production	What Changed
copilot/how-tos/set-up/install-copilot-extension.md	fpt ghec	fpt ghec

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

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