DOCSP-23209 correct pipeline stage in syntax (#1230)

Author: kanchana-mongodb

source/reference/operator/aggregation/unionWith.txt

@@ -221,7 +221,7 @@ the pipeline.
    :emphasize-lines: 5-9
 
    [{
-     "$lookup": {
+     "$unionWith": {
        "coll": <collection-name>,
        "pipeline": [{
          "$search": {