HDFS-13161. Update comment in start-dfs.sh to mention correct variable for secure datanode user. Contributed by Ajay Kumar.

hadoop-hdfs-project/hadoop-hdfs/src/main/bin/start-dfs.sh

@@ -28,7 +28,7 @@
 #    if hdfs_subcmd_user is not defined, error
 #
 # For secure daemons, this means both the secure and insecure env vars need to be
-# defined.  e.g., HDFS_DATANODE_USER=root HADOOP_SECURE_DN_USER=hdfs
+# defined.  e.g., HDFS_DATANODE_USER=root HDFS_DATANODE_SECURE_USER=hdfs
 #
 
 ## @description  usage info