fix IterableDataset doc. test=document_fix (#32544)

python/paddle/fluid/dataloader/dataset.py

@@ -97,10 +97,10 @@ class IterableDataset(Dataset):
         .. code-block:: python
 
             import numpy as np
-            from paddle.io import Dataset
+            from paddle.io import IterableDataset
             
             # define a random dataset
-            class RandomDataset(Dataset):
+            class RandomDataset(IterableDataset):
                 def __init__(self, num_samples):
                     self.num_samples = num_samples