Bug 10976: Fix typo itemtype vs itemtypes

Bug 10976: Fix typo itemtype vs itemtypes

When creating a new report, the limit step let you choose
itype|itemtype.
But it should be itype|itemtypes.
If you try to create a guided report and you choose this option as a
limit, you will get a warning on saving it:
  Errors found when processing parameters for report:
  itype: The authorized value category (itemtype) you selected does not exist.

Looking at the code: reports/guided_reports.pl l.657
  elsif ( $authorised_value eq "itemtypes" ) {

Test plan:
0/ Don't apply the patch
1/ Create a guided report, select Koha itemtype as a limit(step 4)
2/ Confirm you get the error when saving
3/ Apply the patch and repeat step 1
4/ There is no error and the report should work correctly

Tested with steps in comment #4/#5, error is gone after applying patch.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>