tweak wording: single member instead of singe mongod

Author: kay-kim

source/tutorial/troubleshoot-sharded-clusters.txt

@@ -25,8 +25,8 @@ can restart and become unavailable without losing any state or data.
 When a :binary:`~bin.mongos` instance starts, it retrieves a copy of the
 :term:`config database` and can begin routing queries.
 
-A Single :binary:`~bin.mongod` Becomes Unavailable in a Shard
--------------------------------------------------------------
+A Single Member Becomes Unavailable in a Shard Replica Set
+----------------------------------------------------------
 
 :doc:`Replica sets </replication>` provide high availability for shards.
 If the unavailable :binary:`~bin.mongod` is a :term:`primary`, then the