[ServiceWorker] Update explainer.md (#620)

Fixed a typo.
You can view, comment on, or merge this pull request online at:

  https://github.com/slightlyoff/ServiceWorker/pull/620

-- Commit Summary --

  * Update explainer.md

-- File Changes --

    M explainer.md (2)

-- Patch Links --

https://github.com/slightlyoff/ServiceWorker/pull/620.patch
https://github.com/slightlyoff/ServiceWorker/pull/620.diff

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/pull/620