Tutorial: create MyGC doesn't fully follow NoGC

In the file `mmtk-core/docs/tutorial/src/mygc/create.md`, under point 3.iii, when creating a new MyGC plan, the `create_plan()` constructor for MyGC should start with the simple one following NoGC, (i.e. without `scheduler`):
```
// Create MyGC plan based on selector
PlanSelector::MyGC => Box::new(crate::plan::mygc::MyGC::new(
    vm_map, mmapper, options,
))
```