[w3c/ServiceWorker] Use undefined union for getRegistration (#1559)

Thanks to https://github.com/heycam/webidl/pull/906.


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You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/ServiceWorker/pull/1559

-- Commit Summary --

  * Use undefined union for getRegistration

-- File Changes --

    M docs/index.bs (2)

-- Patch Links --

https://github.com/w3c/ServiceWorker/pull/1559.patch
https://github.com/w3c/ServiceWorker/pull/1559.diff

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