servant-docs: Wrong markdowntype javascript for json

## Issue

The markdown docs generated for `json` request values, define the code block syntax type as `javascript`. I think it should be `json` instead.

## Cause

https://github.com/haskell-servant/servant/blob/83bbc6d5208d2a121490624551e32af83536d1e2/servant-docs/src/Servant/Docs/Internal.hs#L776

## Examples

https://github.com/haskell-servant/servant/search?q=javascript