corrected spelling in CHANGES.txt for MAPREDUCE-4397 entry.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-1.1@1439266 13f79535-47bb-0310-9956-ffa450edef68

CHANGES.txt

@@ -6,7 +6,7 @@ Release 1.1.2 - 2012.12.07
 
   NEW FEATURES
 
-    MAPREUDCE-4397. Introduce HADOOP_SECURITY_CONF_DIR for task-controller.
+    MAPREDUCE-4397. Introduce HADOOP_SECURITY_CONF_DIR for task-controller.
     (Yu Gao via llu)
 
     HADOOP-8561. Introduce HADOOP_PROXY_USER for secure impersonation in child