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.