Perform the where(...).first to find_by(...) refactoring.

This refactoring was automated using the command: bundle exec "ruby refactorings/where_dot_first_to_find_by/app.rb"

vendor/gems/discourse_task/lib/discourse_task/plugin.rb

@@ -59,7 +59,7 @@ module DiscourseTask
     module TopicsControllerMixin
 
       def complete
-        topic = Topic.where(id: params[:topic_id]).first
+        topic = Topic.find_by(id: params[:topic_id])
         guardian.ensure_can_complete_task!(topic)
 
         Topic.transaction do