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
(cherry picked from commit 618d056)

Author: MoteHue

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

@@ -76,7 +76,7 @@ for Ceph includes the following steps:
    .. path /etc/kolla/config/glance.conf
    .. code-block:: ini
 
-      [GLOBAL]
+      [DEFAULT]
       show_image_direct_url = True
 
 .. warning::