Fix typo in Spark SQL docs.

Author: Andy Konwinski <andykonwinski@gmail.com>

Closes #3611 from andyk/patch-3 and squashes the following commits:

7bab333 [Andy Konwinski] Fix typo in Spark SQL docs.

(cherry picked from commit 15cf3b0125fe238dea2ce13e703034ba7cef477f)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>