Update source/administration/sharded-clusters.txt

Author: kstirman

source/administration/sharded-clusters.txt

@@ -15,7 +15,7 @@ consists of the following components:
   of the your database collections.
 
 - :ref:`Config servers <sharding-config-server>`. Each config server is
-  a :program:`mongod` instances that holds metadata about the cluster.
+  a :program:`mongod` instance that holds metadata about the cluster.
   The metadata maps :term:`chunks <chunk>` to shards.
 
 - :ref:`mongos instances <sharding-mongos>`. The :program:`mongos`