Fix docs review apps URL
What does this MR do?
Fixes the docs review apps URL that is present in the MR widget.
This should have been the same as https://gitlab.com/gitlab-org/gitlab/-/blob/d33cde7e0986707a572f3c4e5cbdc55a03a1dd26/.gitlab/ci/docs.gitlab-ci.yml#L16-21.
Found out when enabling review apps for the Operator gitlab-org/cloud-native/gitlab-operator!680 (merged).