[docs] fix #each in window-bindings example

[kootstra-rene]

The #each should use the layers as defined in the <script> section so that the parallax works correctly when the layers array is changed

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Tests

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[benmccann]

Here's the example in this PR: https://svelte.dev/examples/svelte-window-bindings

There's a tutorial with basically the same contents which matches this PR: https://svelte.dev/tutorial/svelte-window-bindings