Fix "cookbook drush not found" error

Either the name of the repository changed from `drush` to `chef-drush` at some point in the past, or everyone's been cloning to a folder named `drush`...

Either way, we should refer to the cookbook by it's correct machine name: `chef-drush` to avoid "cookbook drush not found" errors when using librarian-chef to manage cookbooks or when cloning this project without renaming the output folder.