Docs wrongly state GHC CLI supports prompt files

[vandenberghev]

Why:

The docs wrongly state that the GitHub Copilot CLI supports prompt files.
In this issue, it is clearly stated that it does not, nor will it ever be.

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

Prompt file supported - as-is yes - to-be no

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.

[github-actions]

How to review these changes 👓

Thank you for your contribution. To review these changes, choose one of the following options:

• Spin up a codespace
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A Hubber will need to deploy your changes internally to review.

Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
copilot/reference/customization-cheat-sheet.md	fpt ghec	fpt ghec

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

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[moshi2049]

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