[MINOR] Renamed variables in SparkKMeans.scala, LocalKMeans.scala and kmeans.py to simplify readability

With the previous syntax it could look like that the reduceByKey sums separately abscissas and ordinates of some 2D points. Perhaps in this way should be easier to understand the example, especially for who is starting the functional programming like me now.

Author: Niccolo Becchi <niccolo.becchi@gmail.com>
Author: pippobaudos <niccolo.becchi@gmail.com>

Closes #5875 from pippobaudos/patch-1 and squashes the following commits:

3bb3a47 [pippobaudos] renamed variables in LocalKMeans.scala and kmeans.py to simplify readability
2c2a7a2 [Niccolo Becchi] Update SparkKMeans.scala