[SPARK-4330][Doc] Link to proper URL for YARN overview

In running-on-yarn.md, a link to YARN overview is here.
But the URL is to YARN alpha's.
It should be stable's.

Author: Kousuke Saruta <[email protected]>

Closes #3196 from sarutak/SPARK-4330 and squashes the following commits:

30baa21 [Kousuke Saruta] Fixed running-on-yarn.md to point proper URL for YARN

Author: sarutak

docs/running-on-yarn.md

@@ -4,7 +4,7 @@ title: Running Spark on YARN
 ---
 
 Support for running on [YARN (Hadoop
-NextGen)](http://hadoop.apache.org/docs/r2.0.2-alpha/hadoop-yarn/hadoop-yarn-site/YARN.html)
+NextGen)](http://hadoop.apache.org/docs/stable/hadoop-yarn/hadoop-yarn-site/YARN.html)
 was added to Spark in version 0.6.0, and improved in subsequent releases.
 
 # Preparations