Update steps.rb

Update comment on `#previous_step?` to be accurate.

lib/wicked/controller/concerns/steps.rb

@@ -35,7 +35,7 @@ def future_step?(step_name)
     current_step_index < step_index_for(step_name)
   end
 
-  # will return true if the last step is the step passed in
+  # will return true if the previous step is the step passed in
   def previous_step?(step_name)
     return false unless current_and_given_step_exists?(step_name)
     (current_step_index - 1)  == step_index_for(step_name)