[SPARK-10782] [PYTHON] Update dropDuplicates documentation

Documentation for dropDuplicates() and drop_duplicates() is one and the same.  Resolved the error in the example for drop_duplicates using the same approach used for groupby and groupBy, by indicating that dropDuplicates and drop_duplicates are aliases.

Author: asokadiggs <asoka.diggs@intel.com>

Closes #8930 from asokadiggs/jira-10782.

python/pyspark/sql/dataframe.py

@@ -931,6 +931,8 @@ class DataFrame(object):
         """Return a new :class:`DataFrame` with duplicate rows removed,
         optionally only considering certain columns.
 
+        :func:`drop_duplicates` is an alias for :func:`dropDuplicates`.
+
         >>> from pyspark.sql import Row
         >>> df = sc.parallelize([ \
             Row(name='Alice', age=5, height=80), \