Fix systemd spelling

[panekj]

If you have doubts how it is spelled, please see https://freedesktop.org/wiki/Software/systemd/ Spelling section

[learn-build-service-prod[bot]]

Learn Build status updates of commit 297942f:

:bulb: Validation status: suggestions

File	Status	Preview URL	Details
WSL/systemd.md	:bulb:Suggestion	View	Details
WSL/toc.yml	:white_check_mark:Succeeded	View

WSL/systemd.md

• Line 51, Column 1: [Suggestion: inaccessible-video-section] Video title is missing. Consider providing a video title.

For more details, please refer to the build report.

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[mattwojo]

Thank you!