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)