docs: remove email from acme default configuration

This is not required any longer since
https://github.com/NixOS/nixpkgs/pull/489983

docs/setup-example.nix

@@ -18,10 +18,8 @@
     })
   ];
 
-  security.acme = {
+  # https://letsencrypt.org/repository/#let-s-encrypt-subscriber-agreement
-    acceptTerms = true;
+  security.acme.acceptTerms = true;
-    defaults.email = "security@example.com";
-  };
 
   # Allow incoming HTTP connections
   networking.firewall.allowedTCPPorts = [ 80 ];