chore(mongo): quote example

apps/emqx_auth_mongo/etc/emqx_auth_mongo.conf

@@ -16,7 +16,7 @@ auth.mongo.type = single
 ##
 ## Value: String
 ##
-## Examples: 127.0.0.1:27017,127.0.0.2:27017...
+## Examples: "127.0.0.1:27017,127.0.0.2:27017,..."
 auth.mongo.server = "127.0.0.1:27017"
 
 ## MongoDB pool size