Make `v-model` code example more readable

[FloEdelmann]

Description of Problem

Currently, the visual separation of the code blocks (JS vs. HTML) doesn't match the logical separation (child component vs. parent component):

Proposed Solution

Split the first code block in two and change the code comments to headings:

(see https://deploy-preview-3069--vuejs.netlify.app/guide/components/v-model.html#under-the-hood)

[netlify[bot]]

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