[Doc] Fix Glance image size limit command

This commit fixes the configuration document which mentions how to
change Glance default image size quota at runtime because we don't have
`openstack registered limit update` command but
`openstack registered limit set` command[1].

[1] https://docs.openstack.org/python-openstackclient/latest/cli/command-objects/registered-limit.html#registered-limit-set

Change-Id: I399685ed1f864f8f1ce7295ed6f83336cfccbd81
1 file changed