Update the README file for the Sidekiq recipe

In here use to say in order to restart use the group name sidekiq_<app_name> and in the templates/defaults/sidekiq.monitrc.erb at the end the group is declare like this group <%= @app_name %>_sidekiq so I change the documentation file only.
Merge pull request #124 from cavi21/master

Update the README file for the Sidekiq recipe