Update part3a.md

JSON.stringify(notes) doesn't convert array `notes` to JSON. It converts it to string.

Author: vchan14

src/content/3/en/part3a.md

@@ -216,7 +216,7 @@ console.log(`Server running on port ${PORT}`)
 
 Let's restart the server (you can shut the server down by pressing _Ctrl+C_ in the console) and let's refresh the browser.
 
-The <i>application/json</i> value in the <i>Content-Type</i> header informs the receiver that the data is in the JSON format. The _notes_ array gets transformed into JSON with the <em>JSON.stringify(notes)</em> method.
+The <i>application/json</i> value in the <i>Content-Type</i> header informs the receiver that the data is in the JSON format. The _notes_ array gets transformed into JSON string format with the <em>JSON.stringify(notes)</em> method. This is necessary because the response.end() method expects a string or a buffer to send as the response body.
 
 When we open the browser, the displayed format is exactly the same as in [part 2](/en/part2/getting_data_from_server/) where we used [json-server](https://github.com/typicode/json-server) to serve the list of notes:
 
@@ -340,7 +340,7 @@ The request is responded to with the [json](http://expressjs.com/en/4x/api.html#
 
 Next, let's take a quick look at the data sent in JSON format.
 
-In the earlier version where we were only using Node, we had to transform the data into the JSON format with the _JSON.stringify_ method:
+In the earlier version where we were only using Node, we had to transform the data into the JSON string format with the _JSON.stringify_ method:
 
 ```js
 response.end(JSON.stringify(notes))