fixed a typo in change log.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/core/branches/branch-0.20@740134 13f79535-47bb-0310-9956-ffa450edef68

CHANGES.txt

@@ -658,7 +658,7 @@ Release 0.19.1 - Unreleased
     HADOOP-5034. NameNode should send both replication and deletion requests
     to DataNode in one reply to a heartbeat. (hairong)
 
-    HADOOP-556. TestHeartbeatHandling uses MiiDFSCluster.getNamesystem()
+    HADOOP-5156. TestHeartbeatHandling uses MiiDFSCluster.getNamesystem()
     which does not exit in branch 0.19 and 0.20. (hairong)
 
 Release 0.19.0 - 2008-11-18