Update docs to use jsonRDD instead of wrong jsonRdd.

Author: Grega Kespret <grega.kespret@gmail.com>

Closes #2479 from gregakespret/patch-1 and squashes the following commits:

dd6b90a [Grega Kespret] Update docs to use jsonRDD instead of wrong jsonRdd.

(cherry picked from commit 56dae30ca70489a62686cb245728b09b2179bb5a)
Signed-off-by: Michael Armbrust <michael@databricks.com>