[v2] Fix an error on README

[seratch]

Summary

This pull request just fixes a minor error in a code example on README.

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.

[codecov]

Codecov Report

Merging #399 into @slack/bolt@next will not change coverage.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           @slack/bolt@next     #399   +/-   ##
=================================================
  Coverage             85.37%   85.37%           
=================================================
  Files                     7        7           
  Lines                   506      506           
  Branches                144      144           
=================================================
  Hits                    432      432           
  Misses                   49       49           
  Partials                 25       25

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[seratch]

I will merge this PR after merging barlock's PRs.

[seratch]

It seems merging this PR doesn't mean requiring barlock to rebase his PRs. This one obviously doesn't have any conflicts. Let me merge this now.