[SPARK-19734][PYTHON][ML] Correct OneHotEncoder doc string to say dropLast

## What changes were proposed in this pull request?
Updates the doc string to match up with the code
i.e. say dropLast instead of includeFirst

## How was this patch tested?
Not much, since it's a doc-like change. Will run unit tests via Jenkins job.

Author: Mark Grover <mark@apache.org>

Closes #17127 from markgrover/spark_19734.