fix non qualified table name in Course#update_enrolled_users

refs CNVS-21900

Change-Id: Id01b29e93545f52b50b6ec38a889a3a620b4e68d
Reviewed-on: https://gerrit.instructure.com/65667
Tested-by: Jenkins
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

app/models/course.rb

@@ -798,7 +798,7 @@ class Course < ActiveRecord::Base
       end
 
       Enrollment.where(:course_id => self).update_all(:updated_at => Time.now.utc)
-      User.where("id IN (SELECT user_id FROM enrollments WHERE course_id=?)", self).
+      User.where(id: Enrollment.where(course_id: self).select(:user_id)).
           update_all(updated_at: Time.now.utc)
     end
   end