box: Fix allow-none annotation
box: Fix allow-none annotation

The syntax for allowing NULL is "allow-none", not "allow none".

collects/clutter,jigpu/clutter,nobled/clutter,spatulasnout/clutter,djdeath/clutter-android,heysion/clutter-clone,djdeath/clutter-android,Distrotech/clutter,collects/clutter,nobled/clutter,djdeath/clutter-multithreaded,collects/clutter,djdeath/clutter-android,jigpu/clutter,rkudiyarov/clutter_osx,Distrotech/clutter,rkudiyarov/clutter_osx,ebassi/clutter,djdeath/clutter-android,GNOME/clutter,dlespiau/clutter,ebassi/clutter,GNOME/clutter,kerrickstaley/clutter-vala,rkudiyarov/clutter_osx,collects/clutter,spatulasnout/clutter,GNOME/clutter,djdeath/clutter-multithreaded,nobled/clutter,djdeath/clutter,ebassi/clutter,Distrotech/clutter,kerrickstaley/clutter-vala,spatulasnout/clutter,djdeath/clutter,djdeath/clutter,GNOME/clutter,Distrotech/clutter,jigpu/clutter,ebassi/clutter,jigpu/clutter,heysion/clutter-clone,djdeath/clutter-multithreaded,Distrotech/clutter,djdeath/clutter-android,spatulasnout/clutter,Distrotech/clutter,djdeath/clutter-multithreaded,jigpu/clutter,dlespiau/clutter,heysion/clutter-clone,rkudiyarov/clutter_osx,collects/clutter,GNOME/clutter,djdeath/clutter,jigpu/clutter,djdeath/clutter,kerrickstaley/clutter-vala,kerrickstaley/clutter-vala,djdeath/clutter,heysion/clutter-clone,ebassi/clutter,heysion/clutter-clone,spatulasnout/clutter,dlespiau/clutter