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 <sarutak@oss.nttdata.co.jp>

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