docs: add missing backticks to introduction-to-nodejs.md (nodejs#7539)

Hello,

I've added missing backticks to `introduction-to-nodejs.md`.

I believe that `statusCode`, `200`, and `Content-Type` are code related values, so they must be wrapped in backticks (inline code blocks).

Signed-off-by: 루밀LuMir <rpfos@naver.com>

apps/site/pages/en/learn/getting-started/introduction-to-nodejs.md

@@ -83,9 +83,9 @@ In this case with:
 res.statusCode = 200;
 ```
 
-we set the statusCode property to 200, to indicate a successful response.
+we set the `statusCode` property to `200`, to indicate a successful response.
 
-We set the Content-Type header:
+We set the `Content-Type` header:
 
 ```js
 res.setHeader('Content-Type', 'text/plain');