ensure consistent code fencing label for JSON examples

[ghost]

I propose we standardize json for code fences in JSON examples to preserve visual consistency.
Here we use json. Here we use javascript. Here we use text. I am not using blame because I am angry, just to show the exact line number.
Without unifying it, you can see the blocks highlighted differently in the guides:

json

text

[acorncom]

@efx this sounds great, would you have time to tackle it as well?

[ghost]

@acorncom sure thing. I presume this should go into master?

[acorncom]

@efx sorry, this got lost in the notification flood on this end. Yep, master works well. If you want to go back and fix previous releases, that's fine as well, but we don't always do that