current sphinx doesn't support "json" lexer <rpjday@...>

recent commit:

commit 517eba4a824ee742b18a37a96ddc442343647ca3
Author: yacovm <yacovm@...>
Date: Sun Apr 22 02:45:30 2018 +0300

[FAB-8832] Service Discovery overview doc

generates diagnostic:

... discovery-overview.rst:67: WARNING: Could not lex literal_block
as "JSON". Highlighting skipped.

google search shows others have noticed the same thing; alleged
solution is to lex it as javascript, unless there's some trick that
needs to be added here. thoughts?