fix syntax of std/json_test.zig
fix syntax of std/json_test.zig

See #663

raulgrell/zig,zig-lang/zig,raulgrell/zig,Dimenus/zig,andrewrk/zig,Dimenus/zig,raulgrell/zig,andrewrk/zig,Dimenus/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig