docs(cdk/drag-drop): fix show scrollbar in preview of the last element (#29450)

(cherry picked from commit e517789)

Author: josuVazquez

src/components-examples/cdk/drag-drop/cdk-drag-drop-sorting/cdk-drag-drop-sorting-example.css

@@ -24,6 +24,7 @@
 }
 
 .cdk-drag-preview {
+  border: none;
   box-sizing: border-box;
   border-radius: 4px;
   box-shadow: 0 5px 5px -3px rgba(0, 0, 0, 0.2),