Fix #598 add async to v2 migration guide global error handler example

[atl-mk]

Summary

Fixes #598

Requirements (place an x in each [ ])

• 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 #599 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #599   +/-   ##
=======================================
  Coverage   83.27%   83.27%           
=======================================
  Files           7        7           
  Lines         598      598           
  Branches      193      193           
=======================================
  Hits          498      498           
  Misses         67       67           
  Partials       33       33           

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[seratch]

Thanks! If you don't mind, could you make the same change to this file? https://github.com/slackapi/bolt-js/blob/%40slack/bolt%402.3.0/docs/_tutorials/ja_migration_v2.md

[seratch]

Alright, I can work on the Japanese doc. I will merge this within a few hours.

[atl-mk]

Thanks! If you don't mind, could you make the same change to this file? https://github.com/slackapi/bolt-js/blob/%40slack/bolt%402.3.0/docs/_tutorials/ja_migration_v2.md

Done

[atl-mk]

Alright, I can work on the Japanese doc. I will merge this within a few hours.

Oh, whoops 🤭

[seratch]

Thanks! I haven't started working on it 😄