Doc: use "an SQL" consistently rather than "a SQL"

Doc: use "an SQL" consistently rather than "a SQL"

Per the precedent set by 04539e73f, adjust article prefixes for "SQL" to
use "an" consistently rather than "a", i.e., "an es-que-ell" rather than
"a sequel".

Both of these are new to v18. Also see b1b13d2b5, d866f0374 and
7bdd489d3.