Fix string representation of MappingNode in flow style

[semihbkgr]

fixes #622

• Describe the purpose for which you created this PR.
• Create test code that corresponds to the modification

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.52%. Comparing base (c331468) to head (35fc253).
Report is 2 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #623      +/-   ##
==========================================
+ Coverage   77.25%   77.52%   +0.26%     
==========================================
  Files          22       22              
  Lines        7914     7831      -83     
==========================================
- Hits         6114     6071      -43     
+ Misses       1378     1347      -31     
+ Partials      422      413       -9     

[goccy]

Please add the test case to parser/parser_test.go

[goccy]

@semihbkgr Sorry, but the indentation got messed up when resolving the conflict. Could you fix just that? Everything else LGTM.

[goccy]

Thank you for your contribution !!