Update file name from worker.js to index.js

[isarikaya]

Summary

This update corrects a documentation inconsistency where the example referenced a file named worker.js, while the accompanying code sample and typical Workers project structure use index.js.

Screenshots (optional)

N/A

Documentation checklist

• Is there a changelog entry?
• The change adheres to the documentation style guide.
• If a larger change, an issue has been opened related to incorrect or outdated information.
• Files which have changed name or location have been allocated redirects.