Doc: Update PL/pgSQL sample function in plpgsql.sgml.

Doc: Update PL/pgSQL sample function in plpgsql.sgml.

The example used to explain 'Looping Through Query Results' uses
pseudo-materialized views.  Replace it with a more up-to-date example
which does the same thing with actual materialized views, which have
been available since PostgreSQL 9.3.

In the passing, change '%' as format specifier instead of '%s' as is used
in other examples in plpgsql.sgml.

Reported-by: Ian Barwick
Author: Ian Barwick
Reviewed-by: Amit Kapila
Backpatch-through: 9.4
Discussion: https://postgr.es/m/9a70d393-7904-4918-c97c-649f6d114b6a@2ndquadrant.com