docs: change Demo link

[gap-editor]

Why is this change needed?

previous link is not working any more (404 error)

Merge Checklist

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PR-Codex overview

This PR updates the README.md file in the packages/hub-nodejs/examples/hello-world directory, primarily correcting a URL and ensuring proper formatting in the instructions.

Detailed summary

• Updated the web version link from https://farcaster-signup-demo-wkulikowski.vercel.app to https://farcaster-signup-demo.vercel.app/.
• Ensured a newline at the end of the file after the command yarn start.

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Latest commit: e390ee2

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