Add missing registry entry for airgap example
@@ -20,6 +20,7 @@ categories and their values:
dockerLibraryRegistry: docker.io/library
e2eRegistry: gcr.io/kubernetes-e2e-test-images
gcRegistry: k8s.gcr.io
+etcdRegistry: quay.io/coreos
privateRegistry: gcr.io/k8s-authenticated-test
sampleRegistry: gcr.io/google-samples
```
