docs: fix undefined isMac in menu example (#19071)

* docs: fix undefined isMac in menu example

* docs: remove ; in example

docs/api/menu.md

@@ -152,9 +152,11 @@ simple template API:
 ```javascript
 const { app, Menu } = require('electron')
 
+const isMac = process.platform === 'darwin'
+
 const template = [
   // { role: 'appMenu' }
-  ...(process.platform === 'darwin' ? [{
+  ...(isMac ? [{
     label: app.name,
     submenu: [
       { role: 'about' },