Change margin for slice description in charts for the dashboard view (a…

…pache#6575)

Changed margin for the slices description in dashboard view, previously the margin was 5px for all the sides but for improve the view just apply margin for top and bottom.

superset/assets/stylesheets/superset.less

@@ -51,7 +51,7 @@ input.form-control {
 
 .slice_description{
     padding: 8px;
-    margin: 5px;
+    margin: 5px 0;
     border: 1px solid #DDD;
     background-color: #F8F8F8;
     border-radius: 5px;