Fixed a typo in docs/running-on-mesos.md

It should be `spark-env.sh` rather than `spark.env.sh`.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #2119 from liancheng/fix-mesos-doc and squashes the following commits:

f360548 [Cheng Lian] Fixed a typo in docs/running-on-mesos.md