[ServiceWorker] Editorial: Reword normative uses of "can" (#795)

In appendix B there is normative text reading:

> An HTTP request to fetch a service worker's script resource can include the following header:

> An HTTP response to a service worker's script resource request can include the following header:

Those should follow RFC2119 and be phrased as MUST or SHOULD or MAY,

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/795