Update index.scss

Author: mfranzke

packages/mdx/src/mini-editor/index.scss

@@ -24,8 +24,8 @@
   color: rgba(255, 255, 255, 0.5);
   min-width: 0;
   border-bottom: 1px solid;
-  border-left-color: transparent;
-  border-top-color: transparent;
+  border-left: 1px solid transparent;
+  border-top: 1px solid transparent;
 }
 
 .ch-editor-tab-active {