docs: Update plugin pwa docs (#2132)

Author: kefranabg

packages/docs/docs/plugin/official/plugin-pwa.md

@@ -46,10 +46,15 @@ module.exports = {
     ['meta', { name: 'msapplication-TileImage', content: '/icons/msapplication-icon-144x144.png' }],
     ['meta', { name: 'msapplication-TileColor', content: '#000000' }]
   ],
-  plugins: ['@vuepress/pwa', {
-      serviceWorker: true,
-      updatePopup: true
-  }],
+  plugins: [
+    [
+      '@vuepress/pwa',
+      {
+        serviceWorker: true,
+        updatePopup: true
+      }
+    ]
+  ],
 }
 ```