xwayland: fix typo to enable transparency
We spent literally hours trying to debug this. Turns out it's a typo.
See https://specifications.freedesktop.org/wm-spec/1.4/ar01s08.html
Fixes https://github.com/swaywm/wlroots/issues/348 Fixes https://github.com/swaywm/sway/issues/2983