Fix a change log error for HADOOP-3639.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/core/branches/branch-0.18@683354 13f79535-47bb-0310-9956-ffa450edef68

CHANGES.txt

@@ -684,7 +684,7 @@ Release 0.18.0 - 2008-08-06
     HADOOP-3635. Uncaught exception in DataBlockScanner. 
     (Tsz Wo (Nicholas), SZE via hairong)
 
-    HADOOP-3539. Exception when closing DFSClient while multiple files are
+    HADOOP-3639. Exception when closing DFSClient while multiple files are
     open. (Benjamin Gufler via hairong)
 
     HADOOP-3572. SetQuotas usage interface has some minor bugs. (hairong)