docs: Fix version that introduced Xen support for ACPI firmware

Xen support for specifying ACPI firmware path was introduced in the
5.9.0 dev cycle, not 5.8.0 as currently indicated by the docs.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

docs/formatdomain.html.in

@@ -364,7 +364,7 @@
         to the ACPI table. The <code>type</code> attribute contains the
         ACPI table type (currently only <code>slic</code> is supported)
         <span class="since">Since 1.3.5 (QEMU)</span>
-        <span class="since">Since 5.8.0 (Xen)</span></dd>
+        <span class="since">Since 5.9.0 (Xen)</span></dd>
     </dl>
 
     <h4><a id="elementsOSContainer">Container boot</a></h4>