[GraphBolt] Fix docstr of `sampled_csc` in `SampledSubgraph`. (#7004)

Co-authored-by: Ubuntu <ubuntu@ip-172-31-0-133.us-west-2.compute.internal>

python/dgl/graphbolt/sampled_subgraph.py

@@ -54,7 +54,7 @@ class SampledSubgraph:
 
         2. Heterogeneous graph.
 
-        sampled_csc = {"A:relation:B": gb.CSCFormatBase(
+        >>> sampled_csc = {"A:relation:B": gb.CSCFormatBase(
         ...     indptr=torch.tensor([0, 1, 2, 3]),
         ...     indices=torch.tensor([0, 1, 2]))}
         >>> print(sampled_csc)