changed vote button icon display mode to fight zoom bugs

lms/static/sass/_discussion.scss

@@ -1423,8 +1423,7 @@ body.discussion {
     color: #333;
 
     .plus-icon {
-      float: left;
-      display: block;
+      display: inline-block;
       width: 10px;
       height: 10px;
       margin: 8px 6px 0 0;