Update 04-$effect.md

[good-dev-student]

	// reference `messages` array length so that this code re-runs whenever it changes
// No need this line this for old svelte syntax you have to refrence like array = array.push(x) 
	//messages.length;

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• 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
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[gterras]

Is this some kind of social experiment?