docs: tweak snapshot example

[Rich-Harris]

While adding to the docs in #12413 I noticed that our $state.snapshot example could be a little clearer:

• it's not actually using $state.snapshot, it's using $state 🤦
• the point being made gets a bit lost in all the punctuation. if we want to show an example of changing a specific property deep inside a frozen array, we should make it realistic, but I think it's much better to get straight to the point
• Prettier borks it up

(Obviously there'll be a merge conflict between this and #12413; can fix that later.)

I'm aware that the example here is meant to be a continuation of the one above, but I'd like to change that too in a follow-up PR.

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