Minor doc: Remove the extra blank line in data types javadoc.

The extra blank line is preventing the first lines from showing up in the package summary page.

Author: Reynold Xin <rxin@databricks.com>

Closes #4955 from rxin/datatype-docs and squashes the following commits:

1621114 [Reynold Xin] Minor doc: Remove the extra blank line in data types javadoc.