fix: offline page precaching due to revision query parameter

[SebastienTainon]

Hello,
Since PR #386 was merged (and possibly a new version of workbox), the offlinePage option of the documentation does not seem to work.
This is because since this PR, the cache identifier for the offline page now contains a WB_REVISION_ query parameter:

When we do caches.match() it tries to get the page without the query parameter and so it gets nothing.

The workbox precaching module documentation page explains how to fix this:

Alternatively, if all you need is the precached Response object, you can call matchPrecache(), which will automatically use the correct cache key and search in the correct cache:

const response = await matchPrecache('/precached-file.html');

This is what I'm doing in this PR, since we already precache the offlinePage.