docs: site-example: update mesh VPN configuration

docs/site-example/site.conf

@@ -96,16 +96,18 @@
   -- },
 
   mesh_vpn = {
-    -- Refer to http://fastd.readthedocs.org/en/latest/ to better understand
-    -- what these options do.
+    -- enabled = true,
+    mtu = 1280,
+
     fastd = {
+      -- Refer to http://fastd.readthedocs.org/en/latest/ to better understand
+      -- what these options do.
+
       -- List of crypto-methods to use.
       methods = {'salsa2012+umac'},
-      -- enabled = true,
       -- configurable = true,
       -- syslog_level = 'warn',
 
-      mtu = 1280,
       groups = {
         backbone = {
           -- Limit number of connected peers to reduce bandwidth.