pgsql: doc: Fix incorrect reference to textout in plpgsql examples

doc: Fix incorrect reference to textout in plpgsql examples

This error has survived for 22 years, and has been introduced by
da63386.

Reported-by: Erwin Brandstetter
Discussion: /messages/by-id/CAGHENJ57wogGOvGXo5LgWYcqswxafLck8ELqHDR+zrkTPgs_OQ@mail.gmail.com
Backpatch-through: 9.5

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8cea55e99b766fd654431f24987a774622594c9b

Modified Files
--------------
doc/src/sgml/plpgsql.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)