docs: refactor example using jsonencode

[suzuki-shunsuke]

jsonencode

• Validate the parameter
• IDE can validate and format the parameter. Here document is a just string, so IDE can't validate and format the content.

[codecov]

Codecov Report

Merging #269 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #269   +/-   ##
=======================================
  Coverage   52.96%   52.96%           
=======================================
  Files         126      126           
  Lines        6062     6062           
=======================================
  Hits         3211     3211           
  Misses       2657     2657           
  Partials      194      194           

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