[SPARK-15966][DOC] Add closing tag to fix rendering issue for Spark monitoring

## What changes were proposed in this pull request?
Adds the missing closing tag for spark.ui.view.acls.groups

## How was this patch tested?
I built the docs locally and verified the changed in browser.

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)
**Before:**
![image](https://cloud.githubusercontent.com/assets/7732317/16135005/49fc0724-33e6-11e6-9390-98711593fa5b.png)

**After:**
![image](https://cloud.githubusercontent.com/assets/7732317/16135021/62b5c4a8-33e6-11e6-8118-b22fda5c66eb.png)

Author: Dhruve Ashar <dhruveashar@gmail.com>

Closes #13719 from dhruve/doc/SPARK-15966.