script: s/Backports/Backport/

when referencing the "Backport" field in tracker.ceph.com, we
used "Backports". but the field is named "Backport" not "Backports".
in this change, we replace "Backports" with "Backport" when it
references this field.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>