docs: fix incorrect plpgsql error message

Change "$1" to "username".

Reported-by: philipp.salvisberg@gmail.com

Discussion: https://postgr.es/m/172112109590.736590.12219129462878821880@wrigleys.postgresql.org

Backpatch-through: 12

doc/src/sgml/plpgsql.sgml

@@ -1260,7 +1260,7 @@ $$ LANGUAGE plpgsql;
      On failure, this function might produce an error message such as
 <programlisting>
 ERROR:  query returned no rows
-DETAIL:  parameters: $1 = 'nosuchuser'
+DETAIL:  parameters: username = 'nosuchuser'
 CONTEXT:  PL/pgSQL function get_userid(text) line 6 at SQL statement
 </programlisting>
     </para>