Docs: Fix example code for team_join event

[preshetin]

Thanks for this lib! When using example from the documentation I ran into a bug. It appeared the bug leaked into the chunk of code.

Summary

This PR improves documentation. The team_join event looks like { user: {id: 'U1234567', ...}, ... } and not like { user: 'U1234567', ... }.

Since this is a trivial change, I figured not to clone the repo and just fork it and create PR directly from the browser. I hope this is fine in such a trivial case.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #886 (5302288) into main (42e731e) will not change coverage.
The diff coverage is n/a.

❗ Current head 5302288 differs from pull request most recent head 5a14a06. Consider uploading reports for the commit 5a14a06 to get more accurate results

@@           Coverage Diff           @@
##             main     #886   +/-   ##
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  Coverage   66.11%   66.11%           
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  Files          13       13           
  Lines        1201     1201           
  Branches      354      354           
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  Hits          794      794           
  Misses        338      338           
  Partials       69       69           

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[mwbrooks]

Hey @preshetin 👋🏻 Thanks for the compliments about Bolt 🙇🏻

And a big thanks for catching this documentation typo! We thought we fixed all of these, so it's great that you caught this one. A pull request from the browser is totally fine and thanks for signing the CA!

[mwbrooks]

🚀 Your update is live: https://slack.dev/bolt-js/concepts#event-listening

Feel free to delete your branch whenever you like. Thanks again for the contribution!