Fix url bug in the hooks documentation examples

[callumgare]

In a hook event.request.url will be a complete URL I believe (https://developer.mozilla.org/en-US/docs/Web/API/Request/url) meaning event.request.url.startsWith('/custom') will never be true. This commit fixes that.

Checklist boilerplate

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
  I haven't bothered with this since it's such a small and hopefully non-controversial change.
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• Ideally, include a test that fails without this PR but passes with it.
  N/A but I have tried out the old example and run into this issue and changing it fixes it

Tests

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  I couldn't get the project to build due to an error I was running into (Error building project - "Cannot find package 'cookie'" #4642) so I couldn't run these but since it's purely a docs change the code base shouldn't be impacted.

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  N/A

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[Mlocik97]

Doesn't we need to jsut add event.url.pathname.startWith() instead? Because url in event object is already Instance of URL. So constructing URL from URL actually does nothing.

[callumgare]

Opps! Quite right you are. I didn't realise there was both an event.request.url and a event.url, one with a string and one with a URL object. I've updated the commit now.