Fix docs of `glib::derived_properties` #2962

Job	Run time
build GTK binaries with gvsbuild	17s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	3m 20s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	3m 14s
build gtk-rs on Windows (gio, true, --features v2_74)	7m 43s
build gtk-rs on Windows (glib, true, --features v2_74)	4m 48s
build gtk-rs on Windows (glib-build-tools, false)	1m 7s
build gtk-rs on Windows (graphene, false)	3m 46s
build gtk-rs on Windows (pango, true, --features v1_50)	3m 4s
build gtk-rs on Windows (pangocairo, true)	2m 58s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	3m 56s
	34m 13s

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