docs: fix "versions" typo in use-copilot-cli.md

[onismosis]

Why:

Was reading the github cli docs and noticed a typo where a statement read as

A custom agent is a specialized versions of Copilot

Clearly a typo. This small PR fixes that.

Closes:

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

• changed the word "versions" to "version" in the use-custom-agents section

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]

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Table of review links

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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/how-tos/copilot-cli/use-copilot-cli.md	fpt ghec	fpt ghec

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

🤖 This comment is automatically generated.

[Sharra-writes]

@onismosis Good catch, thanks for the fix!

[github-actions]

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