Update 40-service-workers.md

[liborvanek]

Fixes a tiny typo in service worker filename.

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[benmccann]

I don't believe this is a typo. If a user sets up a project using JS instead of TS then the extension will be .js.

Perhaps this would be addressed by #9486

I think we can probably close this PR

[liborvanek]

Yes, #9486 will fix that. I missed that.
Closing this and thanks for the awesome work you all put into Kit!