HADOOP-17479. Fix the examples of hadoop config prefix (#4197)

Signed-off-by: Akira Ajisaka <aajisaka@apache.org>
(cherry picked from commit 40a8b9a6a54181b58d1770dd779d637041a631ce)

hadoop-common-project/hadoop-common/src/site/markdown/DownstreamDev.md

@@ -300,7 +300,7 @@ that conflicts with a property defined by Hadoop can lead to unexpected and
 undesirable results. Users are encouraged to avoid using custom configuration
 property names that conflict with the namespace of Hadoop-defined properties
 and thus should avoid using any prefixes used by Hadoop,
-e.g. hadoop, io, ipc, fs, net, file, ftp, kfs, ha, file, dfs, mapred,
+e.g. hadoop, io, ipc, fs, net, ftp, ha, file, dfs, mapred,
 mapreduce, and yarn.
 
 ### Logging Configuration Files