Bug 18766: Remove itemtype-related warnings from ArticleRequests.t

Bug 18766: Remove itemtype-related warnings from ArticleRequests.t

This patch fixes a small mistake on that test file (itype vs. itemtype
in itemtypes table)

To test:
- Run:
  $ sudo koha-shell kohadev
 k$ cd kohaclone
 k$ prove t/db_dependent/ArticleRequests.t
=> FAIL: itemtype-related warning displayed
- Apply the patch
- Run
 k$ prove t/db_dependent/ArticleRequests.t
=> SUCCESS: No warnings, YAY!
- Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>