docs: consistently capitalize DOM in the Svelte 5 migration guide (#13957)

[looshch]

Make the casing of the word ‘DOM’ in the Svelte 5 migration guide consistent.

#13957

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[looshch]

@benmccann thanks for the approval! Based on your change to the PR title, should i update the first line of my commit as well? It has a full description in the commit body, but i’m not sure what’s the Svelte’s approach on commit titles