Bug 10889: fix comment describing biblioitems.serial

Bug 10889: fix comment describing biblioitems.serial

biblioitems.serial is not a foreign key; it's a Boolean indicating
whether the biblio record is a serial, which in turn influences how
serial items are displayed.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Joy Nelson <joy@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit f22431c2a1415627c9155ee52b5a19ecef01e9ed)
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>