[DOC] Fixes formatting typo in SQL programming guide

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Author: Cheng Lian <lian@databricks.com>

Closes #3498 from liancheng/fix-sql-doc-typo and squashes the following commits:

865ecd7 [Cheng Lian] Fixes formatting typo in SQL programming guide

(cherry picked from commit 2a4d389f70b2066b1ac32b081bef44e61fefb03c)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>