[SQL] Minor edits to sql programming guide.

Author: Henry Cook <hcook@eecs.berkeley.edu>

Closes #2316 from hcook/sql-docs and squashes the following commits:

373f94b [Henry Cook] Minor edits to sql programming guide.

(cherry picked from commit 26bc7655de18ab0191ded3f75cb77bc756dc1c03)
Signed-off-by: Michael Armbrust <michael@databricks.com>