[SPARK-16553][DOCS] Fix SQL example file name in docs

## What changes were proposed in this pull request?

Fixes a typo in the sql programming guide

## How was this patch tested?

Building docs locally

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #14208 from shivaram/spark-sql-doc-fix.