[SPARK-18918][DOC] Missing </td> in Configuration page

### What changes were proposed in this pull request?
The configuration page looks messy now, as shown in the nightly build:
https://people.apache.org/~pwendell/spark-nightly/spark-master-docs/latest/configuration.html

Starting from the following location:

![screenshot 2016-12-18 00 26 33](https://cloud.githubusercontent.com/assets/11567269/21292396/ace4719c-c4b8-11e6-8dfd-d9ab95be43d5.png)

### How was this patch tested?
Attached is the screenshot generated in my local computer after the fix.
[Configuration - Spark 2.2.0 Documentation.pdf](https://github.com/apache/spark/files/659315/Configuration.-.Spark.2.2.0.Documentation.pdf

)

Author: gatorsmile <gatorsmile@gmail.com>

Closes #16327 from gatorsmile/docFix.

(cherry picked from commit c0c9e1d2)
Signed-off-by: Sean Owen <sowen@cloudera.com>