add example bash doc and force json on payload, issue #248

[ProgHaj]

Add an example of usage from curl-cli for the full example at https://flask-restx.readthedocs.io/en/latest/example.html and force json usage in payload, according to suggestions in https://github.com/python-restx/flask-restx/issues/248.

[codecov[bot]]

Codecov Report

Merging #289 (ee6bd95) into master (41b3b59) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #289   +/-   ##
=======================================
  Coverage   96.87%   96.87%           
=======================================
  Files          20       20           
  Lines        2724     2724           
=======================================
  Hits         2639     2639           
  Misses         85       85           

Impacted Files	Coverage Δ
flask_restx/api.py	96.69% <100.00%> (ø)

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