Fix syntax of hexadecimal-digits in chapter-02.md

[Anyplace7803]

PR Summary

Fixes an error in the syntax for hexadecimal-digits:
hexadecimal-digit decimal-digits -> hexadecimal-digit hexadecimal-digits

PR Checklist

• Descriptive Title: This PR's title is a synopsis of the changes it proposes.
• Summary: This PR's summary describes the scope and intent of the change.
• Contributor's Guide: I have read the contributor's guide.
• Style: This PR adheres to the style guide.

[learn-build-service-prod]

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[learn-build-service-prod]

Learn Build status updates of commit e90c819:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/docs-conceptual/lang-spec/chapter-02.md	✅Succeeded	View (>=powershell-5.1)

For more details, please refer to the build report.

[sdwheeler]

Thanks for the fix