SPARK-3318: Documentation update in addFile on how to use SparkFiles.get

Rather than specifying the path to SparkFiles we need to use the filename.

Author: Holden Karau <holden@pigscanfly.ca>

Closes #2210 from holdenk/SPARK-3318-documentation-for-addfiles-should-say-to-use-file-not-path and squashes the following commits:

a25d27a [Holden Karau] Update the JavaSparkContext addFile method to be clear about using fileName with SparkFiles as well
0ebcb05 [Holden Karau] Documentation update in addFile on how to use SparkFiles.get to specify filename rather than path