Decrease 2px bottom border to 1px

IPython/html/static/tree/less/tree.less

@@ -205,6 +205,10 b' ul#new-menu {'
 
                     .list_item {
                         border-bottom: 1px solid #ddd;
+
+                        &:last-child {
+                            border-bottom: 0px;
+                        }
                     }
                 }
             }