Modify magic string "json" to OutputFormatJSON

[yangyanqing]

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.(Just correct a magic string)

• Wrote tests (Just correct a magic string)

• Updated relevant documentation (docs/)

• Added entries in PENDING.md with issue #

• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #3840 into develop will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #3840      +/-   ##
===========================================
+ Coverage    60.92%   60.94%   +0.01%     
===========================================
  Files          192      192              
  Lines        14323    14323              
===========================================
+ Hits          8727     8729       +2     
+ Misses        5027     5025       -2     
  Partials       569      569

[jackzampolin]

Thank you for fixing that! @yangyanqing!