[SPARK-10331] [MLLIB] Update example code in ml-guide

* The example code was added in 1.2, before `createDataFrame`. This PR switches to `createDataFrame`. Java code still uses JavaBean.
* assume `sqlContext` is available
* fix some minor issues from previous code review

jkbradley srowen feynmanliang

Author: Xiangrui Meng <meng@databricks.com>

Closes #8518 from mengxr/SPARK-10331.