Streaming programming guide typos

Fix a bad Java code sample and a broken link in the streaming programming guide.

Author: Clément MATHIEU <clement@unportant.info>

Closes #1286 from cykl/streaming-programming-guide-typos and squashes the following commits:

b0908cb [Clément MATHIEU] Fix broken URL
9d3c535 [Clément MATHIEU] Spark streaming requires at least two working threads (scala version was OK)

(cherry picked from commit fdc4c112e7c2ac585d108d03209a642aa8bab7c8)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>