External Ceph in kolla-ansible

Under the Glance section (https://docs.openstack.org/kolla-ansible/zed/reference/storage/external-ceph-guide.html#glance), to enable copy-on-write, the section name is wrong : it should be `[DEFAULT]` instead of `[GLOBAL]`.

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- [x] This doc is inaccurate in this way: ______
- [ ] This is a doc addition request.
- [x] I have a fix to the document that I can paste below including example: input and output.

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-----------------------------------
Release: 15.1.1.dev33 on 2023-03-24 15:08:41
SHA: 13ced71e0d92a852d7aa3a41b1d6b4c6c9cf9b6b
Source: https://opendev.org/openstack/kolla-ansible/src/doc/source/reference/storage/external-ceph-guide.rst
URL: https://docs.openstack.org/kolla-ansible/zed/reference/storage/external-ceph-guide.html