docs: Fix the GBoxed registration example
docs: Fix the GBoxed registration example

The registration function for serialization and deserialization has been
replaced by two separate functions before releasing 0.10, but I forgot to
update the example in the documentation.

brauliobo/json-glib,ebassi/json-glib,ebassi/json-glib,oerdnj/json-glib,Distrotech/json-glib,brauliobo/json-glib,brauliobo/json-glib,Distrotech/json-glib,oerdnj/json-glib,Distrotech/json-glib,frida/json-glib,oerdnj/json-glib,oerdnj/json-glib,GNOME/json-glib,frida/json-glib,ebassi/json-glib,robtaylor/json-glib-gvariant,robtaylor/json-glib-gvariant,GNOME/json-glib