Doc/Read.me miss leading

[BuhtigithuB]

Hello,

Here : https://github.com/boxcutter/ubuntu/blob/master/README.md
You can read :

The variable HEADLESS can be set to run Packer in headless mode. Set HEADLESS := true, the default is false.

But here to me it kind of set to None or Null :
https://github.com/boxcutter/ubuntu/blob/master/ubuntu.json#L201

Would it be better to cope with the doc to explicitly set this flag to "false"??

Can send a PR

Thanks