doc: Fixed link

Author: Weijun-H

datafusion/physical-plan/src/repartition/on_demand_repartition.rs

@@ -19,6 +19,8 @@
 //! partitions to M output partitions based on a partitioning scheme, optionally
 //! maintaining the order of the input rows in the output. The operator is similar to the [`RepartitionExec`]
 //! operator, but it doesn't distribute the data to the output streams until the downstreams request the data.
+//!
+//! [`RepartitionExec`]: https://docs.rs/datafusion/latest/datafusion/physical_plan/repartition/struct.RepartitionExec.html
 
 use std::pin::Pin;
 use std::sync::Arc;