Gtime sample code update

[greggTime]

Summary

Changing payload to body in EN and JA examples. This will work with both commands and other events so it will be applicable in more cases. It will also cut down on some confusion around trying to look for trigger id in the payload object when dealing with block actions.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #417 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #417   +/-   ##
======================================
  Coverage    81.5%   81.5%           
======================================
  Files           7       7           
  Lines         530     530           
  Branches      152     152           
======================================
  Hits          432     432           
  Misses         70      70           
  Partials       28      28

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[seratch]

@greggTime Thanks for taking the time to improve the docs. It looks good to me!
Could you sign our CLA? https://cla-assistant.io/slackapi/bolt?pullRequest=417

[seratch]

Thanks for your contribution! I'll make a PR for the @slack/bolt@next branch later.