Doc: remove duplicate "a" from func.sgml

Author: Shinya Kato
Discussion: https://postgr.es/m/76c01275776749a167f49379ebec57f1@oss.nttdata.com
Backpatch-through: 15, where that change was introduced

doc/src/sgml/func.sgml

@@ -18033,7 +18033,7 @@ FROM
         or array, but if it is <literal>CONDITIONAL</literal> it will not be
         applied to a single array or object. <literal>UNCONDITIONAL</literal>
         is the default.
-        If the result is a a scalar string, by default the value returned will have
+        If the result is a scalar string, by default the value returned will have
         surrounding quotes making it a valid JSON value. However, this behavior
         is reversed if <literal>OMIT QUOTES</literal> is specified.
         The <literal>ON ERROR</literal> and <literal>ON EMPTY</literal>