Update the TPM doc

Change-Id: I9b93a3f7b1a6a8a523dfa131fb45c9aa877ad6fd
(cherry picked from commit c9be709ba482f5dcff1c915243d9d2ffcafb8f3e)
diff --git a/doc/source/admin/emulated-tpm.rst b/doc/source/admin/emulated-tpm.rst
index 5c1ea34..58f836d 100644
--- a/doc/source/admin/emulated-tpm.rst
+++ b/doc/source/admin/emulated-tpm.rst
@@ -59,14 +59,15 @@
 Image configuration
 -------------------
 
-You can also configure the vTPM on an image using the same properties. For example,
-to configure an image to use the TPM 2.0 with CRB model:
+You can also configure the vTPM on an image using the ``hw_tpm_version`` and
+``hw_tpm_model`` image metadata properties. For example, to configure an image
+to use the TPM 2.0 with CRB model:
 
 .. code-block:: console
 
  $ openstack image set <image-name-or-uuid> \
-     --property hw:tpm_version=2.0 \
-     --property hw:tpm_model=tpm-crb
+     --property hw_tpm_version=2.0 \
+     --property hw_tpm_model=tpm-crb
 
 This can be useful if you need to enable the vTPM feature without having operator
 access to the cloud or for specific images such as Windows versions that require