Updated a few lines in the Launching Spark on YARN Section

docs/running-on-yarn.md

@@ -17,9 +17,10 @@ Java system properties or environment variables not managed by YARN, they should
 Spark application's configuration (driver, executors, and the AM when running in client mode).
 
 There are two deploy modes that can be used to launch Spark applications on YARN. In yarn-cluster mode, the Spark driver runs inside an application master process which is managed by YARN on the cluster, and the client can go away after initiating the application. In yarn-client mode, the driver runs in the client process, and the application master is only used for requesting resources from YARN.
+(Default: --deploy-mode client)
+
+Unlike in Spark standalone and Mesos mode, in which the master's address is specified in the "master" parameter, in YARN mode the ResourceManager's address is picked up from the Hadoop configuration. Thus, the master parameter is yarn. 
 
-Unlike in Spark standalone and Mesos mode, in which the master's address is specified in the "master" parameter, in YARN mode the ResourceManager's address is picked up from the Hadoop configuration. Thus, the master parameter is simply “yarn-client” or “yarn-cluster”.
-
 To launch a Spark application in yarn-cluster mode:
 
     ./bin/spark-submit --class path.to.your.Class --master yarn-cluster [options] <app jar> [app options]