[GraphBolt] Fix doc string of DiskBasedFeature (#7314)

Co-authored-by: Ubuntu <ubuntu@ip-172-31-2-128.us-west-2.compute.internal>

python/dgl/graphbolt/impl/torch_based_feature_store.py

@@ -250,7 +250,9 @@ class DiskBasedFeature(Feature):
     >>> import torch
     >>> from dgl import graphbolt as gb
     >>> torch_feat = torch.arange(10).reshape(2, -1)
-    >>> feature = gb.DiskBasedFeature(torch_feat)
+    >>> pth = "path/to/feat.npy"
+    >>> np.save(pth,torch_feat)
+    >>> feature = gb.DiskBasedFeature(pth)
     >>> feature.read(torch.tensor([0]))
     tensor([[0, 1, 2, 3, 4]])
     >>> feature.size()