DOCSP-15247 fix Atlas agg pipeline link

Author: andf-mongodb

source/reference/operator/aggregation/search.txt

@@ -9,10 +9,11 @@ the field or fields in an Atlas collection. The fields must be covered
 by an :atlas:`Atlas Search 
 </reference/atlas-search/index-definitions/>` index. See:
 
-- :atlas:`Atlas Search Index 
+- :atlas:`Index Definitions
   </reference/atlas-search/index-definitions/>` to learn more about 
   creating and managing Atlas Search indexes.
-- :atlas:`$search Aggregation Pipeline Stage` to learn more about the 
+- :atlas:`$search Aggregation Pipeline Stage
+  </reference/atlas-search/query-syntax/>` to learn more about the 
   ``$search`` pipeline stage syntax and usage.
 - :atlas:`$search Operators </reference/atlas-search/operators/>` to 
   learn more about the ``$search`` aggregation pipeline stage