PB-108 apply display:flex css to immediate children only in column line.

Author: paras89

app/code/Magento/PageBuilder/view/adminhtml/web/css/source/content-type/column-line/_default.less

@@ -19,7 +19,7 @@
         z-index: 26;
     }
 
-    .element-children {
+   > .element-children {
         display: flex;
         width: 100%;
     }