Swap CreatedAtAction for CreatedAtRoute (#1671)

`GetTodo` is the route name so we should use `CreatedAtRoute`.
The tutorial also mention using `CreatedAtRoute`

aspnet/tutorials/first-web-api/sample/src/TodoApi/Controllers/TodoController.cs

@@ -38,7 +38,7 @@ namespace TodoApi.Controllers
                 return BadRequest();
             }
             TodoItems.Add(item);
-            return CreatedAtAction("GetTodo", new { id = item.Key }, item);
+            return CreatedAtRoute("GetTodo", new { id = item.Key }, item);
         }
 
         [HttpPut("{id}")]