fix ($plugin-pwa): popup zindex conflict (fix #2642) (#2643)

Changed z-index from 2 to 3 to avoid conflict with div[class*="language-"]::before elements

Author: smwbtech

packages/@vuepress/plugin-pwa/lib/SWUpdatePopup.vue

@@ -93,7 +93,7 @@ export default {
   background: #fff;
   box-shadow: 0 4px 16px rgba(0, 0, 0, 0.5);
   text-align: center;
-  z-index: 2;
+  z-index: 3;
 }
 
 .sw-update-popup > button {