YARN-6343. Docker docs MR example is broken (Contributed by Prashant Jha via Daniel Templeton)

(cherry picked from commit 14a39907c636420800210b984f2f78b639c8382c)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-site/src/site/markdown/DockerContainers.md

@@ -269,8 +269,8 @@ To submit the pi job to run in Docker containers, run the following commands:
 
 ```
     vars="YARN_CONTAINER_RUNTIME_TYPE=docker,YARN_CONTAINER_RUNTIME_DOCKER_IMAGE=hadoop-docker"
-    hadoop jar hadoop-examples.jar -Dyarn.app.mapreduce.am.env=$vars \
-        -Dmapreduce.map.env=$vars -Dmapreduce.reduce.env=$vars pi 10 100
+    hadoop jar hadoop-examples.jar pi -Dyarn.app.mapreduce.am.env=$vars \
+        -Dmapreduce.map.env=$vars -Dmapreduce.reduce.env=$vars 10 100
 ```
 
 Note that the application master, map tasks, and reduce tasks are configured