Fix documentation about state verification option

[stophecom]

Summary

This PR fixes the documentation about the usage of the stateVerification option.
On a side note. This probably should be fixed here https://slack.dev/bolt-js/concepts as well.

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 #1168 (4df6cc4) into main (02ca6e1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1168   +/-   ##
=======================================
  Coverage   71.71%   71.71%           
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  Files          15       15           
  Lines        1354     1354           
  Branches      402      402           
=======================================
  Hits          971      971           
  Misses        312      312           
  Partials       71       71           

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[srajiang]

Good catch, thanks for your contribution @stophecom ! 🙌
With this merged, the page you linked in the PR description should be updated.