[MINOR][DOCS] Added Missing back slashes

## What changes were proposed in this pull request?

When studying spark many users just copy examples on the documentation and paste on their terminals
and because of that the missing backlashes lead them run into some shell errors.

The added backslashes avoid that problem for spark users with that behavior.

## How was this patch tested?

I generated the documentation locally using jekyll and checked the generated pages

Author: Daniel Santana <mestresan@gmail.com>

Closes #11699 from danielsan/master.

docs/submitting-applications.md

@@ -30,7 +30,7 @@ dependencies, and can support different cluster managers and deploy modes that S
 
 {% highlight bash %}
 ./bin/spark-submit \
-  --class <main-class>
+  --class <main-class> \
   --master <master-url> \
   --deploy-mode <deploy-mode> \
   --conf <key>=<value> \
@@ -92,8 +92,8 @@ run it with `--help`. Here are a few examples of common options:
 ./bin/spark-submit \
   --class org.apache.spark.examples.SparkPi \
   --master spark://207.184.161.138:7077 \
-  --deploy-mode cluster
-  --supervise
+  --deploy-mode cluster \
+  --supervise \
   --executor-memory 20G \
   --total-executor-cores 100 \
   /path/to/examples.jar \
@@ -120,8 +120,8 @@ export HADOOP_CONF_DIR=XXX
 ./bin/spark-submit \
   --class org.apache.spark.examples.SparkPi \
   --master mesos://207.184.161.138:7077 \
-  --deploy-mode cluster
-  --supervise
+  --deploy-mode cluster \
+  --supervise \
   --executor-memory 20G \
   --total-executor-cores 100 \
   http://path/to/examples.jar \