Add missing word "operator"

Author: matthurne

source/applications/aggregation.txt

@@ -184,7 +184,7 @@ the aggregation pipeline, you may want to optimize the operation
 by avoiding scanning the entire collection whenever possible.
 
 If your aggregation operation requires only a subset of the data in a
-collection, use the :agg:pipeline:`$match` to restrict which items go
+collection, use the :agg:pipeline:`$match` operator to restrict which items go
 in to the top of the pipeline, as in a query. When placed early in a
 pipeline, these :agg:pipeline:`$match` operations use suitable indexes
 to scan only the matching documents in a collection.