docs: (Tutorial) Change spread and reassignment to .push()

[lts20050703]

Svelte 5 now uses a runtime reactivity system, so no need to use the Svelte 4 workaround of spreading and reassigning anymore.

I'm debating whether we should also change the filter to a findIndex + splice, like technically it would save the need of allocating a new array, but it seems like a big change comparing to a simple change to .push()

I love Svelte, I'm doing the tutorial again to keep up to date with Svelte 5 & SvelteKit features.

This is my first ever PR, sorry if I did something wrong, please let me know and I will try my best to fix it!

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