Correct glance.conf example in external Ceph guide

Under the Glance section of the external Ceph guide, to enable
copy-on-write the config heading should be ``[DEFAULT]``, rather than
``[GLOBAL]``. For reference, see the Glance docs:
https://docs.openstack.org/glance/latest/configuration/glance_api.html#DEFAULT.show_image_direct_url

Closes-Bug: #2017643
Change-Id: I2d77e01637ecae0bd10319eac4bc75340dd6f61f

doc/source/reference/storage/external-ceph-guide.rst

@@ -128,7 +128,7 @@ for multistore:
   .. path /etc/kolla/config/glance.conf
   .. code-block:: ini
 
-     [GLOBAL]
+     [DEFAULT]
      show_image_direct_url = True
 
 .. warning::