docs: fix wsl file system notation

[mikaello]

Fixes https://github.com/MicrosoftDocs/WSL/issues/1753

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

Learn Build status updates of commit 5593a9f:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
WSL/tutorials/wsl-containers.md	:white_check_mark:Succeeded	View

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[craigloewen-msft]

Closed this on the issue. The usage here is correct!

[mikaello]

So it is correct to show single slash?