Fix spelling and punctuation issues in GitHub CLI quickstart docs

[NA-V10]

Why:

Improves the clarity and correctness of the GitHub CLI quickstart documentation.
There were two small issues affecting readability:

• Inconsistent spelling: “onscreen” → “on-screen”
• Extra space before a period (“codespace .”)

These are documentation-quality fixes that enhance consistency and alignment with GitHub Docs style guidelines.

What's being changed:

• Replaced “onscreen” with the stylistically correct “on-screen”.
• Removed an unintended extra space before a period in the Codespaces section.
• No functional or technical changes were made — documentation only.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. (These changes are grammar/punctuation only; no technical accuracy concerns.)
• 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.

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[github-actions]

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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
github-cli/github-cli/quickstart.md	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.19 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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[Sharra-writes]

@NA-V10 Thanks for opening a PR! We will definitely take the typo fix, although our docs prefer "onscreen" to "on-screen," so we'll leave that the way it is for consistency.

[github-actions]

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