Dockerfile.*: Fix "etcd is distributed" -> "etcd is a distributed"

Correcting a typo from 2f10964 (version: openshift-v4.0, 2018-11-29).

Author: wking

Dockerfile.openshift

@@ -16,5 +16,5 @@ ENTRYPOINT ["/usr/bin/etcd"]
 COPY --from=builder /go/src/go.etcd.io/etcd/bin/{etcd,etcdctl} /usr/bin/
 
 LABEL io.k8s.display-name="etcd server" \
-      io.k8s.description="etcd is distributed key-value store which stores the persistent master state for Kubernetes and OpenShift." \
+      io.k8s.description="etcd is a distributed key-value store which stores the persistent master state for Kubernetes and OpenShift." \
       maintainer="Sam Batschelet <[email protected]>"

Dockerfile.rhel

@@ -16,5 +16,5 @@ ENTRYPOINT ["/usr/bin/etcd"]
 COPY --from=builder /go/src/go.etcd.io/etcd/bin/{etcd,etcdctl} /usr/bin/
 
 LABEL io.k8s.display-name="etcd server" \
-      io.k8s.description="etcd is distributed key-value store which stores the persistent master state for Kubernetes and OpenShift." \
+      io.k8s.description="etcd is a distributed key-value store which stores the persistent master state for Kubernetes and OpenShift." \
       maintainer="Sam Batschelet <[email protected]>"