Fix type in put_nb_static_property stubgen

[mrexodia]

This was generating invalid .pyi files with code like this:

nb::class_<NbDatabase>(m, "Database")
	.def_prop_ro_static("current", &NbDatabase::get_current, "Get the current database")

Actually I think this might be a missing static_assert in def_prop_ro_static because my getter has this signature:

static NbDatabase get_current();

[wjakob]

@mrexodia This is breaking the test suite. Could you instead create an issue with a self-contained reproducer demonstrating the problem you encountered?