docs: fix webContents 'new-window' event handler sample (#18097)

docs/api/web-contents.md

@@ -138,11 +138,16 @@ new [`BrowserWindow`](browser-window.md). If you call `event.preventDefault()` a
 instance, failing to do so may result in unexpected behavior. For example:
 
 ```javascript
-myBrowserWindow.webContents.on('new-window', (event, url) => {
+myBrowserWindow.webContents.on('new-window', (event, url, frameName, disposition, options) => {
   event.preventDefault()
-  const win = new BrowserWindow({ show: false })
+  const win = new BrowserWindow({
+    webContents: options.webContents, // use existing webContents if provided
+    show: false
+  })
   win.once('ready-to-show', () => win.show())
-  win.loadURL(url)
+  if (!options.webContents) {
+    win.loadURL(url) // existing webContents will be navigated automatically
+  }
   event.newGuest = win
 })
 ```