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

doc/src/sgml/ddl.sgml

@@ -5479,7 +5479,7 @@ CREATE FUNCTION get_color_note (rainbow) RETURNS text AS
   </para>
 
   <para>
-   On the other hand, for a SQL-language function or procedure whose body
+   On the other hand, for an SQL-language function or procedure whose body
    is written in SQL-standard style, the body is parsed at function
    definition time and all dependencies recognized by the parser are
    stored.  Thus, if we write the function above as