[SPARK-1701] [PySpark] remove slice terminology from python examples

Author: Matthew Farrellee <matt@redhat.com>

Closes #2304 from mattf/SPARK-1701-partition-over-slice-for-python-examples and squashes the following commits:

928a581 [Matthew Farrellee] [SPARK-1701] [PySpark] remove slice terminology from python examples