Fix typo in xdg-activation-v1
Noticed during review in !98 (comment 1003427), but not changed at the time. Noticed again in https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3090#note_1606895.
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Noticed during review in !98 (comment 1003427), but not changed at the time. Noticed again in https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3090#note_1606895.