fix hover text on course content notification

fixes #9722

it should indicate that it is for changes to course pages
and assignments.

test plan:
- go to profile > notifications and hover over the "Course
  content" text
- it should say "for changes to course pages and assignments"

Change-Id: I52de267b00787d475c83e66c1143ad0de8ea3000
Reviewed-on: https://gerrit.instructure.com/12688
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>

app/models/notification.rb

@@ -612,7 +612,7 @@ class Notification < ActiveRecord::Base
     when 'Announcement'
       t(:announcement_description, "For new announcements")
     when 'Course Content'
-      t(:course_content_description, "For changes to course pages")
+      t(:course_content_description, "For changes to course pages and assignments")
     when 'Files'
       t(:files_description, "For new files")
     when 'Discussion'