Remove directories from checksums file

currently checksum file shows as this:

1d15c3c86ab26d43622721ea31f0b57beb499fabde638170d5a2dc8ade93d54b  ./release/images.txt
a9442d93eba0738836e2630622905c26aa066d10f562b1f93f4ff4965d914924  ./release/images.tar.gz
9f2e2dfb34a6ed3a658a876737e36c48b34e2bd1f28fc15cb0ea3a27a629dc01  ./release/repositories.tar.gz
bd2288b91a05dec9e0cc105ed0ada4c635cadcb36da94330e46ab086401948ac  ./release/velero-2.31.8-bb.2_checksums.txt

passing through sed shows as this:

1d15c3c86ab26d43622721ea31f0b57beb499fabde638170d5a2dc8ade93d54b  images.txt
a9442d93eba0738836e2630622905c26aa066d10f562b1f93f4ff4965d914924  images.tar.gz
9f2e2dfb34a6ed3a658a876737e36c48b34e2bd1f28fc15cb0ea3a27a629dc01  repositories.tar.gz
bd2288b91a05dec9e0cc105ed0ada4c635cadcb36da94330e46ab086401948ac  velero-2.31.8-bb.2_checksums.txt

I think this is prettier