demo: update media dir in cms example config (#1796)

exampleSite/static/admin/config.yml

@@ -1,8 +1,8 @@
 backend:
   name: git-gateway
   branch: master
-media_folder: 'static/img/'
-public_folder: 'img'
+media_folder: 'static/media/'
+public_folder: 'media'
 collections:
   - name: authors
     label: Authors