Trivial: fix image format typo
Trivial: fix image format typo

The Glance V1 image format list contains "VDHX", which is a typo.
This change fixes it, using the correct format name, which is "VHDX".

Luckily, this seems to be used only as part of a help string.

Change-Id: I392f25b3ee0ee9ac6024e1670053191e4bba937a

openstack/python-glanceclient,openstack/python-glanceclient