style: change dropdown item heading padding (#2482)

Author: gtsiolis

packages/@vuepress/theme-default/components/DropdownLink.vue

@@ -137,7 +137,7 @@ export default {
       h4
         margin 0.45rem 0 0
         border-top 1px solid #eee
-        padding 0.45rem 1.5rem 0 1.25rem
+        padding 1rem 1.5rem 0.45rem 1.25rem
       .dropdown-subitem-wrapper
         padding 0
         list-style none