Apply maintainer suggestion

app/services/releases/update_service.rb

@@ -12,12 +12,12 @@ module Releases
       return error("Milestone(s) not found: #{inexistent_milestones.join(', ')}", 400) if inexistent_milestones.any?
 
       if param_for_milestone_titles_provided?
-        @previous_milestones = release.milestones.map(&:title)
+        previous_milestones = release.milestones.map(&:title)
         params[:milestones] = milestones
       end
 
       if release.update(params)
-        success(tag: existing_tag, release: release, milestones_updated: milestones_updated?)
+        success(tag: existing_tag, release: release, milestones_updated: milestones_updated?(previous_milestones))
       else
         error(release.errors.messages || '400 Bad request', 400)
       end
@@ -33,10 +33,10 @@ module Releases
       params.except(:tag).empty?
     end
 
-    def milestones_updated?
+    def milestones_updated?(previous_milestones)
       return false unless param_for_milestone_titles_provided?
 
-      @previous_milestones.to_set != release.milestones.map(&:title)
+      previous_milestones.to_set != release.milestones.map(&:title)
     end
   end
 end