Fix warnings in json_client example

[ashtum]

No description provided.

[cppalliance-bot]

An automated preview of the documentation is available at https://2930.beastdocs.prtest.cppalliance.org/libs/beast/doc/html/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://2930.beastdocs.prtest.cppalliance.org/libs/beast/doc/html/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.10%. Comparing base (3ddcf4e) to head (c3093e7).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2930   +/-   ##
========================================
  Coverage    93.10%   93.10%           
========================================
  Files          177      177           
  Lines        13656    13656           
========================================
  Hits         12714    12714           
  Misses         942      942           

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