By default Ubuntu doesn’t specify a ServerName in the Apache configuration, because it doesn’t know what the name of your server is. It tries a reverse lookup on your IP address, which returns nothing, so it just has to use the IP address as the ServerName.
To fix it, either add a ServerName directive outside of any virtual host – e.g. in /etc/apache2/httpd.conf, or set up a reverse DNS response for your primary IP address – in this case, 127.0.1.1
It’s perfectly fine to ignore it also.