docs: remove mention of non-existent publicModule option

[teemingc]

part of #13385

This PR removes the recommendation to configure the name of the _env.js file through the publicModule option since it doesn't exist. It also corrects the name of the public module file from _env.js to env.js (without the underscore) since that's the name the codebase actually uses.

https://github.com/search?q=repo:sveltejs/kit%20ir%7D/env.js&type=code

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Preview: https://svelte-dev-git-preview-kit-13406-svelte.vercel.app/