docs: fix broken link to the WebClient

[WilliamBergamin]

Summary

This PR aims to fix a broken link in our documentation. It was identified in #1127

Testing

1. Click on the link

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 90.96%. Comparing base (4fce092) to head (3fb1274).
Report is 1 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #1235 +/- ##
=======================================
Coverage 90.96% 90.96% =======================================
Files 222 222 Lines 7501 7501 =======================================
Hits 6823 6823 Misses 678 678 

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