Wrong command syntax '--sort-by-required'

[chakatz]

I'm a first-time user, but it seems to me that
--sort-by-required should be --sort-by required
It is wrong in several places in the readme.

https://github.com/cytopia/docker-terraform-docs/blob/master/README.md

I ran the command below to figure out the proper syntax
docker run --rm -v $(pwd):/data cytopia/terraform-docs terraform-docs help

[cytopia]

Good point. This is a relict from older versions when arguments where named differently. I will make it a PR to clean up the legacy commands.