[4518] One reservation example updated.

doc/guide/dhcp4-srv.xml

@@ -2705,6 +2705,8 @@ It is merely echoed by the server
     <para>Vendor specific options can be reserved in a similar manner:</para>
 
     <screen>
+{
+    "subnet4": [ {
         "reservations": [
         {
             "hw-address": "aa:bb:cc:dd:ee:ff",
@@ -2719,7 +2721,9 @@ It is merely echoed by the server
                 "space": "vendor-4491",
                 "data": "10.1.1.202,10.1.1.203"
             } ]</userinput>
-} ]</screen>
+        } ]
+    } ]
+}</screen>
 
 <para>
  Options defined on host level have the highest priority. In other words,