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

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/10/static/acronyms.html
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