[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

Author: masayukig

doc/source/configuration.rst

@@ -719,7 +719,7 @@ or at runtime via:
 
 ::
 
-    openstack --os-cloud devstack-system-admin registered limit update \
+    openstack --os-cloud devstack-system-admin registered limit set \
       --service glance --default-limit 5000 --region RegionOne image_size_total
 
 .. _arch-configuration: