Fixed download link on fileset show page
I cherry picked the commit https://gitlab.ruhr-uni-bochum.de/FDM/rdm-system/rdms/-/commit/e6021d67f3c996a547b99576de4e5f60420751ac from the merge request https://gitlab.ruhr-uni-bochum.de/FDM/rdm-system/rdms/-/merge_requests/110, to point to the correct branch.