Doc: use "an SQL" instead of "a SQL"

Doc: use "an SQL" instead of "a SQL"

Although which is correct depends entirely on whether you pronounce SQL
as "ess-que-ell" or "sequel", we have standardized on the former in our
user-facing documentation, so use the correct article according to that
pronunciation.

Discussion: https://postgr.es/m/CAApHDvp3osQwQam+wNTp9BdhP+QfWO6aY6ZTixQQMfM-UArKCw@mail.gmail.com