[SPARK-13312][MLLIB] Update java train-validation-split example in ml-guide

Response to JIRA https://issues.apache.org/jira/browse/SPARK-13312.

This contribution is my original work and I license the work to this project.

Author: JeremyNixon <jnixon2@gmail.com>

Closes #11199 from JeremyNixon/update_train_val_split_example.