formatdomain: Update <loader/> example to match the rest

At the beginning when I was inventing <loader/> attributes and
<nvram/> I've introduced this @readonly attribute to the loader
element. It accepted values 'on' and 'off'. However, later, during the
review process, that has changed to 'yes' and 'no', but the example
XML snippet wasn't updated, so while the description is correct, the
example isn't.
Reported-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

docs/formatdomain.html.in

@@ -102,7 +102,7 @@
   ...
   <os>
     <type>hvm</type>
-    <loader readonly='on' type='rom'>/usr/lib/xen/boot/hvmloader</loader>
+    <loader readonly='yes' type='rom'>/usr/lib/xen/boot/hvmloader</loader>
     <nvram template='/usr/share/OVMF/OVMF_VARS.fd'>/var/lib/libvirt/nvram/guest_VARS.fd</nvram>
     <boot dev='hd'/>
     <boot dev='cdrom'/>