updated java code blocks in spark SQL guide such that ctx will refer to ...

...a JavaSparkContext and sqlCtx will refer to a JavaSQLContext

Author: Yadid Ayzenberg <yadid@media.mit.edu>

Closes #932 from yadid/master and squashes the following commits:

f92fb3a [Yadid Ayzenberg] updated java code blocks in spark SQL guide such that ctx will refer to a JavaSparkContext and sqlCtx will refer to a JavaSQLContext