[w3c/ServiceWorker] Update link to caching API (#992)

The current one leads to a 404 page.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/ServiceWorker/pull/992

-- Commit Summary --

  * Update link to caching API

-- File Changes --

    M explainer.md (2)

-- Patch Links --

https://github.com/w3c/ServiceWorker/pull/992.patch
https://github.com/w3c/ServiceWorker/pull/992.diff

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