docs: uniCloud db aggregate count

docs/uniCloud/cf-database.md

@@ -12336,9 +12336,12 @@ let res = await db.collection('items').aggregate()
   .end()
 ```
 返回结果如下：
-```js
-{
-  "expensiveCount": 3
+```js
+// res = 
+{
+  "data": [{
+    "expensiveCount": 3
+  }]
 }
 ```