Fix incorrect class name in backfill migration

Author: MrSerth

db/migrate/20181119161514_add_user_to_proxy_exercise.rb

@@ -13,6 +13,6 @@ def change
     add_column :proxy_exercises, :public, :boolean, null: false, default: false
 
     internal_user = InternalUser.find_by(id: 46) || InternalUser.first
-    ProxyExercise.update(user_id: internal_user&.id || 1, user_type: internal_user.class.name)
+    ProxyExercise.update(user_id: internal_user&.id || 1, user_type: internal_user.class.name.demodulize)
   end
 end