`FORMAT` has incorrect result

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

### 2. What did you expect to see? (Required)

```
mysql> SELECT FORMAT(1000/3, 5);
+-------------------+
| FORMAT(1000/3, 5) |
+-------------------+
| 333.33333 |
+-------------------+
1 row in set (0.01 sec)
```

### 3. What did you see instead (Required)

```
mysql> SELECT FORMAT(1000/3, 5);
+-------------------+
| FORMAT(1000/3, 5) |
+-------------------+
| 333.33330 |
+-------------------+
1 row in set (0.01 sec)
```

### 4. What is your TiDB version? (Required)