Certificate installation instructions for DNS plugins do not install certificates

As pointed out in https://community.letsencrypt.org/t/along-with-the-flag-i-apache-where-do-i-need-to-put-it/98518, the instruction generator says that to obtain a certificate with a DNS plugin and to install it with the apache/nginx plugin, you just need to select your plugin at https://certbot.eff.org/docs/using.html#dns-plugins, running one of the example commands, but adding `-i ` to the command line. This doesn't work because all the example commands as of writing this contain `certonly` meaning that the installer will not be used to install the certificate.

I think the easiest, quickest fix for this for now is to also say to not include `certonly` in the command you run.