Add :html option to config.format in acceptance_helper

At present, only option is :open_api, but this does not
produce output that is easily readable by user.

When the problem with the :json option is fixed see issue
#382
I suggest that :json option is added as well, users can
see output using raddocs.

Author: Chris Drane

example/spec/acceptance_helper.rb

@@ -3,7 +3,7 @@
 require 'rspec_api_documentation/dsl'
 
 RspecApiDocumentation.configure do |config|
-  config.format = [:open_api]
+  config.format = [:open_api, :html]
   config.curl_host = 'http://localhost:3000'
   config.api_name = "Example App API"
   config.api_explanation = "API Example Description"