Update source/administration/replica-sets.txt

Fix typo.

Author: milkie

source/administration/replica-sets.txt

@@ -961,7 +961,7 @@ error itself will resemble the following.
 
    exception 11000 E11000 duplicate key error index: local.slaves.$_id_  dup key: { : ObjectId('<object ID>') } 0ms 
 
-This is benign error and does not affect replication operations on
+This is a benign error and does not affect replication operations on
 the :term:`secondary` or :term:`slave`.
 
 To prevent the error from appearing, drop the ``local.slaves``