doc: Clean up unnecessary left vertical lines
doc: Clean up unnecessary left vertical lines

This is a counterpart of the glance doc patch [0].
This commit cleans up unexpected vertical lines at left side like [1].
For more detail context, see the commit mesasge of [0].

[0] https://review.opendev.org/682791
[1] https://docs.openstack.org/glance/latest/configuration/glance_cache.html#glance_store.swift_upload_buffer_dir

Change-Id: If1571631f4d24bc0550e2f42c49c9529451d74d2

openstack/glance_store,openstack/glance_store