[SPARK-19396][DOC] JDBC Options are Case In-sensitive

### What changes were proposed in this pull request?
The case are not sensitive in JDBC options, after the PR https://github.com/apache/spark/pull/15884 is merged to Spark 2.1.

### How was this patch tested?
N/A

Author: gatorsmile <gatorsmile@gmail.com>

Closes #16734 from gatorsmile/fixDocCaseInsensitive.