Fix Learn PWA stale while revalidate example

[kabalage]

The code in this example does not work: Learn PWA! > 008 Serving > Stale while revalidate, because the networkFetch promise does not resolve to a Response.

I fixed the example matching the promise chaining style that's used in the rest of the document. (An async/await version would be nicer though.)

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[stale[bot]]

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