fix docs

Author: adriangb

datafusion/optimizer/src/push_down_sort.rs

@@ -56,6 +56,8 @@ use datafusion_expr::{Expr, LogicalPlanContext, ScanOrdering, SortExpr};
 /// Sort: test.a ASC NULLS LAST
 ///   TableScan: test preferred_ordering=[test.a ASC NULLS LAST]
 /// ```
+/// 
+/// [`TableProvider`]: https://docs.rs/datafusion/latest/datafusion/datasource/trait.TableProvider.html
 #[derive(Default, Debug)]
 pub struct PushDownSort {}