Add demo

Src/Asp.NetCore2/SqlSeverTest/Demo/Demo2_Updateable.cs

@@ -55,6 +55,16 @@ namespace OrmTest
             var result6 = db.Updateable(updateObjs).WhereColumns(it => new { it.Id }).ExecuteCommand();//update List<Class> by id
 
 
+            var list = db.Queryable<Order>().OrderBy(it => it.Id).Take(2).ToList();
+            if (list.Count >= 2)
+            {
+                list[0].Price = 10;
+                list[1].Price = 2;
+                var result69 =
+                db.Updateable(list)
+                .PublicSetColumns(it => it.Price, "+")
+                .ExecuteCommand();
+            }
 
 
             /*** 2.by expression ***/