Add :json option back to config.format

[rakvium]

in example/spec/acceptance_helper.rb
to make raddocs for the example project work again

There was an exception on /docs route:

Errno::ENOENT at /docs/
No such file or directory @ rb_sysopen - doc/api/index.json

After adding the :json format back and running rake docs:generate, /docs started working again