docs: remove class names from fiddle examples (#20596)

* docs: remove original class names in customize menus example

* docs: remove original class names in shortcuts example

* lint: run standard

docs/fiddles/screen/fit-screen/main.js

@@ -17,4 +17,4 @@ app.on('ready', () => {
 
   mainWindow = new BrowserWindow({ width, height })
   mainWindow.loadURL('https://electronjs.org')
-})
+})