Update svelte code snippet for manaually saving state in remembering-state.jsx

[joekrump]

Update the svelte code snippet for manually saving state. @inertiajs/svelte does not export router and @inertiajs/core does, so I've updated the snippet to import from @inertiajs/core.

Perhaps @inertia/svelte should expose route, however, this change is focused on keeping the docs accurate.

Here's what I get if I try to use import { router } from "@inertiajs/svelete";: