[SPARK-8043] [MLLIB] [DOC] update NaiveBayes and SVM examples in doc

jira: https://issues.apache.org/jira/browse/SPARK-8043



I found some issues during testing the save/load examples in markdown Documents, as a part of 1.4 QA plan

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #6584 from hhbyyh/naiveDocExample and squashes the following commits:

a01a206 [Yuhao Yang] fix for Gaussian mixture
2fb8b96 [Yuhao Yang] update NaiveBayes and SVM examples in doc

(cherry picked from commit 43adbd56)
Signed-off-by: Xiangrui Meng <meng@databricks.com>