docs: Fix a regression for a quoted-attribute example

[andy0130tw]

The code example is inconsistent with the text:

An expression might include characters that would cause syntax highlighting to fail in regular HTML, so quoting the value is permitted. The quotes do not affect how the value is parsed: [...]

But the attribute in the code example is not quoted at all:

<button disabled={number !== 42}>...</button>

This small PR fixes the code example by placing back missing quotation marks.

It is a regression, as the docs before site redesign explain it correctly; see https://github.com/sveltejs/svelte/blob/v3.59.2/site/content/docs/03-template-syntax.md.

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[PuruVJ]

Thank you!