docs: Fixes swapped logic for readonly function bindings

[a-theriault]

Read-only bindings should have the SET value assigned to null, not the GET.

e.g. this:
<input type="checkbox" bind:indeterminate={null, () => true} />

...fails, while this:

<input type="checkbox" bind:indeterminate={() => true, null} />

...works as expected.
(note: ts doesn't love this, prefers: <input type="checkbox" bind:indeterminate={() => true, () => null} />)

Question: should I also add explicit wording for bind:indeterminate since it no longer works without explicitly defining a getter/setter:

<input type="checkbox" bind:indeterminate={true} />

Fails.

<input type="checkbox" bind:indeterminate={() => true, null} />

is the only way this binding currently works in Svelte5.

Alternatively, should support for indeterminate={true} be a feat: ticket?

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