[AssetMapper] Change the structure of the imortmap file

Author: javiereguiluz

frontend/asset_mapper.rst

@@ -215,7 +215,7 @@ This adds the ``bootstrap`` package to your ``importmap.php`` file::
         // ...
 
         'bootstrap' => [
-            'url' => 'https://cdn.jsdelivr.net/npm/bootstrap@5.3.0/+esm',
+            'version' => '5.3.0',
         ],
     ];