typo: correction

Author: Sam Kleinman

source/applications/replication.txt

@@ -258,7 +258,7 @@ behavior <replica-set-read-preference-behavior>`. See also the
 
       When connecting to a :program:`mongos` instance older 2.2, using
       a client that supports read preference modes,
-      :readmode:`primaryPrefered` will send queries to secondaries.
+      :readmode:`primaryPreferred` will send queries to secondaries.
 
 .. readmode:: secondary