Update opener.mdx - fix missing ] in example (#3448)

Author: 7underlines

src/content/docs/plugin/opener.mdx

@@ -159,6 +159,7 @@ Lastly, an example on how to add permissions for the exact `https://tauri.app` U
         {
           "url": "custom:*"
         }
+      ]
     }
   ]
 }