Fix typo in the document (#4636)

<!--
Thanks for your contribution! please review
https://github.com/alibaba/GraphScope/blob/main/CONTRIBUTING.md before
opening an issue.
-->

## What do these changes do?

<!-- Please give a short brief about these changes. -->

## Related issue number

<!-- Are there any issues opened that will be resolved by merging this
change? -->

Fixes

Author: Mrhs121

docs/storage_engine/graphar.md

@@ -108,7 +108,7 @@ You can also save a subgraph in GraphAr format using the `save_to` function with
 
 ```python
 import graphscope
-from graphscoped.dataset import load_ldbc
+from graphscope.dataset import load_ldbc
 
 # initialize a session
 sess = graphscope.session(cluster_type="hosts")
@@ -130,7 +130,7 @@ selector = {
 }
 
 # save the subgraph to GraphAr format
-r = g.save_to(
+r = graph.save_to(
     "/tmp/ldbc_subgraph_graphar/",
     format="graphar",
     selector=selector,