make events in Japanese document clearer

[Shoryu-N]

This pull request fixes #427

Category (place an x in each of the [ ])

• 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 (place an x in each [ ])

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• 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.

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[seratch]

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[codecov]

Codecov Report

Merging #429 (d55e461) into main (8c978ae) will not change coverage.
The diff coverage is n/a.

❗ Current head d55e461 differs from pull request most recent head 020149a. Consider uploading reports for the commit 020149a to get more accurate results

@@           Coverage Diff           @@
##             main     #429   +/-   ##
=======================================
  Coverage   91.36%   91.36%           
=======================================
  Files         167      167           
  Lines        5491     5491           
=======================================
  Hits         5017     5017           
  Misses        474      474           

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