HDFS-5018. Misspelled DFSConfigKeys#DFS_NAMENODE_STALE_DATANODE_INTERVAL_DEFAULT in javadoc of DatanodeInfo#isStale(). Contributed by Ted Yu.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1505603 13f79535-47bb-0310-9956-ffa450edef68