fix invalid lang tags (#19513)

docs/tutorial/first-app.md

@@ -17,7 +17,7 @@ Node.js application. The starting point is a `package.json` that is identical
 to that of a Node.js module. A most basic Electron app would have the following
 folder structure:
 
-```text
+```plaintext
 your-app/
 ├── package.json
 ├── main.js