Fix cli about missing comma

[NA-V10]

Why:

Improves the grammar and readability of the “About GitHub CLI” documentation.
A missing comma after an introductory clause made the sentence grammatically incorrect.

What's being changed:

• Added a comma after the introductory phrase “If you tend to work on the command line,”.
• This helps align the page with GitHub Docs style guidelines.
• Documentation-only change.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy (grammar-only fix).
• The changes meet documentation fundamentals.
• All CI checks are passing.

[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
github-cli/github-cli/about-github-cli.md	fpt ghec ghes@ 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.18 3.17 3.16 3.15 3.14
github-cli/github-cli/quickstart.md	fpt ghec ghes@ 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.18 3.17 3.16 3.15 3.14
github-cli/github-cli/using-multiple-accounts.md	fpt ghec ghes@ 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.18 3.17 3.16 3.15 3.14

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

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