[SPARK-19302][DOC][MINOR] Fix the wrong item format in security.md

## What changes were proposed in this pull request?

In docs/security.md, there is a description as follows.

```
 steps to configure the key-stores and the trust-store for the standalone deployment mode is as
 follows:
 * Generate a keys pair for each node
 * Export the public key of the key pair to a file on each node
 * Import all exported public keys into a single trust-store
```

According to markdown format, the first item should follow a blank line.

## How was this patch tested?

Manually tested.

Following captures are rendered web page before and after fix.

* before
![before](https://cloud.githubusercontent.com/assets/4736016/22136731/b358115c-df19-11e6-8f6c-2f7b65766265.png)

* after
![after](https://cloud.githubusercontent.com/assets/4736016/22136745/c6366ff8-df19-11e6-840d-e7e894218f9c.png)

Author: sarutak <sarutak@oss.nttdata.co.jp>

Closes #16653 from sarutak/SPARK-19302.

docs/security.md

@@ -82,6 +82,7 @@ Key-stores can be generated by `keytool` program. The reference documentation fo
 [here](https://docs.oracle.com/javase/7/docs/technotes/tools/solaris/keytool.html). The most basic
 steps to configure the key-stores and the trust-store for the standalone deployment mode is as
 follows:
+
 * Generate a keys pair for each node
 * Export the public key of the key pair to a file on each node
 * Import all exported public keys into a single trust-store