Update `etc` example to not use a real config file

A user noticed the example for `hosts`, took the `mode` permissions literally, and ended up with surprising behavior on their system. Updating the documentation to not reference a real config file which might have real permissions requirements.

nixos/modules/system/etc/etc.nix

···
       type = types.loaOf types.optionSet;
       default = {};
       example = literalExample ''
-        { hosts =
+        { example-configuration-file =
             { source = "/nix/store/.../etc/dir/file.conf.example";
               mode = "0440";
             };