HDFS-7867. Update action param from 'start' to 'prepare' in rolling upgrade ( Contributed by J.Andreina) Updated JIRA id.

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -330,7 +330,7 @@ Release 2.8.0 - UNRELEASED
     HDFS-5356. MiniDFSCluster should close all open FileSystems when shutdown()
     (Rakesh R via vinayakumarb)
 
-    HDFS-7869. Update action param from "start" to "prepare" in rolling upgrade
+    HDFS-7867. Update action param from "start" to "prepare" in rolling upgrade
     javadoc (J.Andreina via vinayakumarb)
 
 Release 2.7.0 - UNRELEASED