Fix the bugs atom feed

`@bug` should be just `bug`. 

Without this, when loading the atom feed, you get this error:

```console
E, [2014-07-09T15:30:38.561285 #2948] ERROR -- : [failsafe_handler] 2014-07-09 15:30:38 -0700 - Original error: (NoMethodError) undefined method `special_file?' for nil:NilClass
E, [2014-07-09T15:30:38.561545 #2948] ERROR -- : [failsafe_handler]   /home/squash/squashapp/releases/20131005033134/app/views/bugs/index.atom.builder:25:in `block (3 levels) in _app_views_bugs_index_atom_builder__2938631946974487145_52828660'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/actionpack-4.0.0/lib/action_view/helpers/atom_feed_helper.rb:196:in `block in entry'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/builder-3.1.4/lib/builder/xmlbase.rb:170:in `call'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/builder-3.1.4/lib/builder/xmlbase.rb:170:in `_nested_structures'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/builder-3.1.4/lib/builder/xmlbase.rb:63:in `tag!'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/builder-3.1.4/lib/builder/xmlbase.rb:88:in `method_missing'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/actionpack-4.0.0/lib/action_view/helpers/atom_feed_helper.rb:181:in `entry'
  /home/squash/squashapp/releases/20131005033134/app/views/bugs/index.atom.builder:20:in `block (2 levels) in _app_views_bugs_index_atom_builder__2938631946974487145_52828660'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/relation/delegation.rb:13:in `each'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/relation/delegation.rb:13:in `each'
  /home/squash/squashapp/releases/20131005033134/app/views/bugs/index.atom.builder:19:in `block in _app_views_bugs_index_atom_builder__2938631946974487145_52828660'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/actionpack-4.0.0/lib/action_view/helpers/atom_feed_helper.rb:123:in `block in atom_feed'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/builder-3.1.4/lib/builder/xmlbase.rb:170:in `call'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/builder-3.1.4/lib/builder/xmlbase.rb:170:in `_nested_structures'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/builder-3.1.4/lib/builder/xmlbase.rb:63:in `tag!'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/builder-3.1.4/lib/builder/xmlbase.rb:88:in `method_missing'
  /home/squash/squashapp/shared/bundle/ruby/2.0.0/gems/actionpack-4.0.0/lib/action_view/helpers/atom_feed_helper.rb:118:in `atom_feed'
  /home/squash/squashapp/releases/20131005033134/app/views/bugs/index.atom.builder:15:in `_app_views_bugs_index_atom_builder__2938631946974487145_52828660'
```