docs: update quick-start.md (#40556)

Updates the Quick Start guide to specify _where_ JavaScript code is supposed to be added. This is more descriptive than just "your file".

docs/tutorial/quick-start.md

@@ -155,7 +155,7 @@ need two Electron modules:
   windows.
 
 Because the main process runs Node.js, you can import these as [CommonJS][commonjs]
-modules at the top of your file:
+modules at the top of your `main.js` file:
 
 ```js
 const { app, BrowserWindow } = require('electron')