style: fix dropdown vertical padding variable substitution (#2287)

Co-authored-by: honglian <[email protected]>

Author: bgpo

components/dropdown/style/index.less

@@ -107,8 +107,8 @@
 
       > a {
         display: block;
-        margin: -5px -@control-padding-horizontal;
-        padding: 5px @control-padding-horizontal;
+        margin: -@dropdown-vertical-padding -@control-padding-horizontal;
+        padding: @dropdown-vertical-padding @control-padding-horizontal;
         color: @text-color;
         transition: all 0.3s;
       }