Removed reference to incubation in Spark user docs.

Author: Reynold Xin <rxin@apache.org>

Closes #2 from rxin/docs and squashes the following commits:

08bbd5f [Reynold Xin] Removed reference to incubation in Spark user docs.
(cherry picked from commit 40e080a68a8fd025435e9ff84fa9280b4aba4dcf)

Conflicts:

	docs/_config.yml