Use ptr::write in example

[bluss]

This example for function State::new_userdata_typed is only correct if MyStruct does not have a destructor. The correct way to move a value into an uninitialized location is std::ptr::write

The given example:

unsafe { *state.new_userdata_typed() = MyStruct::new(...); }
state.set_metatable_from_registry("MyStruct");