ibusManager: fix ibus launch error because of wrong method name

Mar 09 09:07:52 pendragon gnome-shell[663]: Failed to launch ibus-daemon: launcher.launch is not a function

It seems that there is no launch method for Gio.SubprocessLauncher, but a spawnv method do this, so it might be a typo.

@carlosg please have a look at this (because git blame tells me that you write those code) and if I am wrong, please close this MR, thanks!