doc: vtpm only support secrets by UUID at this point

Support by usage name can be considered separately (with a 'usage'
attribute?).
Reviewed-by: NStefan Berger <stefanb@linux.ibm.com>
Reviewed-by: NChristophe de Dinechin <dinechin@redhat.com>
Cc: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>

docs/formatsecret.html.in

@@ -334,8 +334,8 @@ Secret value set
       of the vTPM.
       The <code>&lt;usage type='vtpm'&gt;</code> element must contain
       a single <code>name</code> element that specifies a usage name
-      for the secret.  The vTPM secret can then be used by UUID or by
-      this usage name via the <code>&lt;encryption&gt;</code> element of
+      for the secret.  The vTPM secret can then be used by UUID
+      via the <code>&lt;encryption&gt;</code> element of
       a <a href="formatdomain.html#elementsTpm">tpm</a> when using an
       emulator.
       <span class="since">Since 5.6.0</span>. The following is an example