[MINOR][DOC] Fix doc for web UI https configuration

## What changes were proposed in this pull request?

Doc about enabling web UI https is not correct, "spark.ui.https.enabled" is not existed, actually enabling SSL is enough for https.

## How was this patch tested?

N/A

Author: jerryshao <sshao@hortonworks.com>

Closes #17147 from jerryshao/fix-doc-ssl.

docs/security.md

@@ -12,7 +12,7 @@ Spark currently supports authentication via a shared secret. Authentication can
 ## Web UI
 
 The Spark UI can be secured by using [javax servlet filters](http://docs.oracle.com/javaee/6/api/javax/servlet/Filter.html) via the `spark.ui.filters` setting
-and by using [https/SSL](http://en.wikipedia.org/wiki/HTTPS) via the `spark.ui.https.enabled` setting.
+and by using [https/SSL](http://en.wikipedia.org/wiki/HTTPS) via [SSL settings](security.html#ssl-configuration).
 
 ### Authentication