registry: fix typo in yaml file

Introduced by previous commit.

Signed-off-by: Daniel Wagner <wagi@kernel.org>

Author: igaw

.github/workflows/registry.yml

@@ -25,7 +25,7 @@ jobs:
         uses: snok/container-retention-policy@3b0972b2276b171b212f8c4efbca59ebba26eceb # v3.0.1
         id: retention
         with:
-          image-names
+          image-names:
             - alpine
             - alpine.staging
             - debian