Fixed DynamicGridDurationColumn Button label

inabox.wpf/DynamicGrid/Columns/EditorColumns/DynamicGridDurationColumn.cs

@@ -36,7 +36,7 @@ public class DynamicGridDurationColumn<TEntity> : DynamicGridMaskColumn<TEntity,
         };
         
         var more = new Button();
-        more.Content = "-";
+        more.Content = "+";
         more.Width = 25;
         more.Click += (sender, args) =>
         {