manifests/bootupd: Drop unnecessary /

It's not supported to pass anything other than `/` now, and
the argument has been optional for some time.

tier-0/bootupd.yaml

@@ -28,4 +28,4 @@ postprocess:
     # Until we have https://github.com/coreos/rpm-ostree/pull/2275
     mkdir -p /run
     # Transforms /usr/lib/ostree-boot into a bootupd-compatible update payload
-    /usr/bin/bootupctl backend generate-update-metadata /
+    /usr/bin/bootupctl backend generate-update-metadata