Docs: adds updated bolt-py image to README

[lukegalbraithrussell]

Summary

The bolt python image was removed from the repo given that the docs.slack.dev site can serve as img hosting too. I forgot to actually update the README img link though. This does so

Testing

Category

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• 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.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.95%. Comparing base (974816e) to head (f6bf8d2).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1352      +/-   ##
==========================================
- Coverage   90.96%   90.95%   -0.02%     
==========================================
  Files         222      222              
  Lines        7507     7507              
==========================================
- Hits         6829     6828       -1     
- Misses        678      679       +1     

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[WilliamBergamin]

Sweet 🙏 🚤