Add missing let (#15746)

docs/tutorial/first-app.md

@@ -109,7 +109,7 @@ const { app, BrowserWindow } = require('electron')
 
 function createWindow () {
   // Create the browser window.
-  win = new BrowserWindow({ width: 800, height: 600 })
+  let win = new BrowserWindow({ width: 800, height: 600 })
 
   // and load the index.html of the app.
   win.loadFile('index.html')