Add JSON to the list of acronyms in the documentation's appendix

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Description:

Hello all,

I propose to add 'JSON' (for JavaScript Object Notation, http://json.org) to
the list of acronyms in the documentation's appendix
(https://www.postgresql.org/account/comments/new/10/acronyms.html/)

Best wishes,
—Torsten