Fixed small running on YARN docs typo

The backslash is needed for multiline command

Author: Vlad <frolvlad@gmail.com>

Closes apache#1158 from frol/patch-1 and squashes the following commits:

e258044 [Vlad] Fixed small running on YARN docs typo

docs/running-on-yarn.md

@@ -128,7 +128,7 @@ For example:
         --num-executors 3 \
         --driver-memory 4g \
         --executor-memory 2g \
-        --executor-cores 1
+        --executor-cores 1 \
         lib/spark-examples*.jar \
         10