Eliminate duplicate font setup instructions in custom-prompt-setup.md

[agniuks]

Noticed this while reading through for new pc setup.
Removed duplicate instructions for setting a Nerd Font in Windows Terminal.

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

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[learn-build-service-prod]

Learn Build status updates of commit a5572bf:

✅ Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/tutorials/custom-prompt-setup.md	✅Succeeded	View

For more details, please refer to the build report.