docs: Fix Effect subclassing section
docs: Fix Effect subclassing section

It still mentions the long since removed "prepare" function.

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