fix: copy the hello world extension spec to the correct place

The hello world extension example copied its spec to the 'containers'
path as a file instead of copying it into the 'containers' path as a
directory.

Signed-off-by: Philipp Sauter <philipp.sauter@siderolabs.com>

examples/hello-world-service/pkg.yaml

@@ -27,4 +27,4 @@ finalize:
   - from: /pkg/manifest.yaml
     to: /
   - from: /pkg/hello-world.yaml
-    to: /rootfs/usr/local/etc/containers
+    to: /rootfs/usr/local/etc/containers/