apacheGH-35073: [R] Minimal build is failing (acero symbol not defined) (apache#35074)

This PR updates the decorator in a function introduced in apache#34685 which inadvertently referenced `arrow` and not `acero`.
* Closes: apache#35073

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>

Author: thisisnic

r/src/arrowExports.cpp

@@ -261,7 +261,7 @@ std::shared_ptr<arrow::Schema> ExecNode_output_schema(
   return node->output_schema();
 }
 
-// [[arrow::export]]
+// [[acero::export]]
 bool ExecNode_has_ordered_batches(const std::shared_ptr<acero::ExecNode>& node) {
   return !node->ordering().is_unordered();
 }