Improve sentence structure

Author: matthurne

source/applications/aggregation.txt

@@ -255,10 +255,10 @@ runs. This pipeline consists of the first :agg:pipeline:`$group` or
 on the results received from the shards.
 
 The :program:`mongos` pipeline merges :agg:pipeline:`$sort` operations
-from the shards. The :agg:pipeline:`$group`, brings any “sub-totals”
+from the shards. The :agg:pipeline:`$group` operation brings in any “sub-totals”
 from the shards and combines them: in some cases these may be
 structures. For example, the :agg:expression:`$avg` expression maintains a
-total and count for each shard; the :program:`mongos` combines these
+total and count for each shard; :program:`mongos` combines these
 values and then divides.
 
 .. [#match-sharding] If an early :agg:pipeline:`$match` can exclude