fix doc tests

Author: alamb

datafusion/expr/src/logical_plan/rewrite.rs

@@ -127,6 +127,8 @@ impl LogicalPlan {
     ///
     /// Inputs include both direct children as well as any embedded subquery
     /// `LogicalPlan`s, for example such as are in [`Expr::Exists`].
+    ///
+    /// [`Expr::Exists`]: crate::expr::Expr::Exists
     pub(crate) fn rewrite_children<F>(&mut self, mut f: F) -> Result<Transformed<()>>
     where
         F: FnMut(Self) -> Result<Transformed<Self>>,