Improve phrasing

Author: matthurne

source/applications/aggregation.txt

@@ -246,7 +246,7 @@ Sharded Operation
 The aggregation framework is compatible with sharded collections.
 
 When operating on a sharded collection, the aggregation pipeline
-splits the pipeline into two parts. The aggregation framework pushes
+is split into two parts. The aggregation framework pushes
 all of the operators up to and including the first
 :agg:pipeline:`$group` or :agg:pipeline:`$sort` to each shard.
 [#match-sharding]_ Then, a second pipeline on the :program:`mongos`