ntpd: Use server instead of servers in openntpd.conf.erb

This matches the behaviour of ISC ntpd.

ntpd/templates/openntpd.conf.erb

@@ -5,13 +5,13 @@
 listen on *
 
 # sync to a single server
-#server ntp.example.org
+<% @ntp_server.each do |server| -%>
+server <%= server %>
+<% end -%>
 
 # use a random selection of NTP Pool Time Servers
 # see http://support.ntp.org/bin/view/Servers/NTPPoolServers
-<% @ntp_server.each do |server| -%>
-servers <%= server %>
-<% end -%>
+#servers pool.ntp.org
 
 # use a specific local timedelta sensor (radio clock, etc)
 #sensor nmea0