chore(react): improve DragSource styles

Author: janryWang

packages/react/src/widgets/DragSourceWidget/styles.less

@@ -59,6 +59,7 @@
     width: 50%;
     min-height: 40px;
     margin: -1px;
+    margin-bottom: 0px;
     flex-grow: 2;
     border: 1px solid var(--dn-panel-border-color);
     color: var(--dn-drag-source-item-color);