docs: fix ADD destination in container-register-disks
With destination set to /disk, it places the image to file /disk. We need to
change it to /disk/ instead, so the image is put under /disk directory.
@@ -63,7 +63,7 @@ Example:
```
cat << END > Dockerfile
FROM scratch
-ADD fedora25.qcow2 /disk
+ADD fedora25.qcow2 /disk/
END
docker build -t vmdisks/fedora25:latest .
docker push vmdisks/fedora25:latest
