schema: remove reference to brctl

ip(8) is the contemporary way of configuring bridges.
Signed-off-by: NJán Tomko <jtomko@redhat.com>

docs/schemas/interface.rng

@@ -130,7 +130,8 @@
               <ref name="virOnOff"/>
             </attribute>
           </optional>
-          <!-- Bridge forward delay (see 'brctl setfd') -->
+          <!-- Bridge forward delay
+               (see 'ip link set <dev> type bridge forward_delay') -->
           <optional v:since="2">
             <attribute name="delay"><ref name="timeval"/></attribute>
           </optional>