arch.md: fix typo

Signed-off-by: Andrew Turner <andrewjamesturner0@gmail.com>
todo-vm.md: update btsync instructions

* Add note on editing service file, now that btsync@<user> is deprecated.
* NOTE: these instructions run a system service not a user service.
  This is because it is run on a headless server where no user logs in

Signed-off-by: Andrew Turner <andrewjamesturner0@gmail.com>