[ServiceWorker] Fix typos, terminology (#843)

I've replaced "ServiceWorker" with "service worker" per [the spec](https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-service-worker).
You can view, comment on, or merge this pull request online at:

  https://github.com/slightlyoff/ServiceWorker/pull/843

-- Commit Summary --

  * Fix typos, terminology

-- File Changes --

    M explainer.md (37)

-- Patch Links --

https://github.com/slightlyoff/ServiceWorker/pull/843.patch
https://github.com/slightlyoff/ServiceWorker/pull/843.diff

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/pull/843