minor fix: aggregate -> aggregate command

Author: stbrody

source/release-notes/2.2.txt

@@ -109,7 +109,7 @@ Aggregation Framework
 
 The aggregation framework makes it possible to do aggregation
 operations without needing to use :term:`map-reduce`. The
-:dbcommand:`aggregate` exposes the aggregation framework, and the
+:dbcommand:`aggregate` command exposes the aggregation framework, and the
 :func:`db.collection.aggregate()` helper in the :program:`mongo` shell
 provides an interface to these operations. Consider the following
 resources for background on the aggregation framework and its use: