Fixes SPARK-5805 : Fix the type error in the final example given in MLlib - Clustering documentation.

Author: Emre Sevinç <emre.sevinc@gmail.com>

Closes #4596 from emres/SPARK-5805 and squashes the following commits:

1029f66 [Emre Sevinç] SPARK-5805 Fixed the type error in documentation.