etcd.conf.yml: use "stderr" for log-outputs

Fix "TestEtcdExampleConfig".

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

etcd.conf.yml.sample

@@ -131,7 +131,7 @@ debug: false
 logger: zap
 
 # Specify 'stdout' or 'stderr' to skip journald logging even when running under systemd.
-log-outputs: [default]
+log-outputs: [stderr]
 
 # Force to create a new one member cluster.
 force-new-cluster: false