doc: update PL/pgSQL sample loop function

Hi

Here:

https://www.postgresql.org/docs/current/plpgsql-control-structures.html#PLPGSQL-RECORDS-ITERATING

we have a sample PL/PgSQL function (dating from at least 7.2) demonstrating
query result loops, which refreshes some pseudo materialized views stored in
a user-defined table.

As we've had proper materialized views since 9.3, I thought it might
be nice to update this with a self-contained sample which can be used
as-is; see attached patch.

(As a side note the current sample function contains a couple of "%s"
placeholders which should be just "%"; a quick search of plpgsql.sgml
shows this is the only place they occur).

Will submit to the next commitfest.

Regards

Ian Barwick

--
Ian Barwick https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services