Replace "for" with "in"

Author: matthurne

source/applications/aggregation.txt

@@ -220,7 +220,7 @@ must fit in memory.
 
 :agg:pipeline:`$group` has similar characteristics: Before any
 :agg:pipeline:`$group` passes its output along the pipeline, it must
-receive the entirety of its input. For the case of :agg:pipeline:`$group`
+receive the entirety of its input. In the case of :agg:pipeline:`$group`
 this frequently does not require as much memory as
 :agg:pipeline:`$sort`, because it only needs to retain one record for
 each unique key in the grouping specification.