text: Title: Japanese: Make "events" in the document even clearer (#412)
  Body: We can apply the updates in #415 to the Japanese version of the documents. See also #412 
  
  ### The page URLs
  
  * https://slack.dev/bolt-python/ja-jp/concepts#basic
  
  ## Requirements
  
  Please read the [Contributing guidelines](https://github.com/slackapi/bolt-python/blob/main/.github/contributing.md) and [Code of Conduct](https://slackhq.github.io/code-of-conduct) before creating this issue or pull request. By submitting, you are agreeing to those rules.
  