docs: "```js" -> "```json" in Markdown of some json blocks

doc/api/http_api.md

@@ -72,7 +72,7 @@ The API is accessible via HTTP. HTTP Requests are in the format /api/$APIVERSION
 ### Response Format
 Responses are valid JSON in the following format:
 
-```js
+```json
 {
   "code": number,
   "message": string,

doc/plugins.md

@@ -50,7 +50,7 @@ There are server hooks, which will be executed on the server (e.g. `expressCreat
 ### Parts
 As your plugins become more and more complex, you will find yourself in the need to manage dependencies between plugins. E.g. you want the hooks of a certain plugin to be executed before (or after) yours. You can also manage these dependencies in your plugin definition file `ep.json`:
 
-```javascript
+```json
 {
   "parts": [
     {