optimize config example file.

conf.sample.yml

@@ -50,7 +50,7 @@
 
 # LogLevel: 'info'              # default: info. can be debug|info|warn|error
 # Log:
-#   Outputs:   ''               # default: stderr, split by ",", you can append files to Outputs if need. example:'stdout,/tmp/test.log'
+#   Outputs: 'stderr'           # default: stderr, split by ",", you can append files to Outputs if need. example:'stderr,/tmp/test.log'
 #   RotationEnable: 0           # default: 0
 #   RotationConfigJson: '{}'    # example: '{"maxsize": 100, "maxage": 0, "maxbackups": 0, "localtime": false, "compress": false}'