Fix the generator

Since the change from JSON to YML, this generator is broken. Update the code to make it work.
Thanks alext. 
I have another question, should `docs_path = 'app/views/govuk_component/docs.json'` be 
`docs_path = 'File.join(Rails.root, 'app', 'views', 'govuk_component', 'docs.yml')'`