[SPARK-2547]:The clustering documentaion example provided for spark 0.9....

I modified a trivial mistake in the MLlib documentation.
I checked that the python sample code for a k-means clustering can correctly run on a EC2 instance.

Author: Yuu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #1590 from yu-iskw/branch-0.9 and squashes the following commits:

06eeb94 [Yuu ISHIKAWA] [SPARK-2547]:The clustering documentaion example provided for spark 0.9.1/docs is having a error