Bug 13850: Fix typo in C4::Ris

Bug 13850: Fix typo in C4::Ris

In C4/Ris.pm:
  @field_list = ('600', '601', '602', '604', '605', '606','607', '608',
  '610', '615', '620', '660'. '661', '670', '675', '676', '680', '686');

Note the
  '660'. '661'
instead of
  '660', '661'

Test plan:
Add a 660 and 661 field to a record and export it in RIS
The 660 and 661 field should be displayed (after "KW").

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>