[ trivial ] fix sharded cluster typo

Author: andf-mongodb

source/release-notes/4.2-downgrade-replica-set.txt

@@ -67,8 +67,7 @@ These include:
    - Ensure that no replica set member is in :replstate:`ROLLBACK` or
      :replstate:`RECOVERING` state.
 
-To downgrade the ``featureCompatibilityVersion`` of your sharded
-cluster:
+To downgrade the ``featureCompatibilityVersion`` of your replica set:
 
 #. Connect a :binary:`~bin.mongo` shell to the |target|.