Use `show_object()` in the code sample

This is the default code sample when a user opens CQ-Editor for the first time:

```python
import cadquery as cq
result = cq.Workplane("XY" ).box(3, 3, 0.5).edges("|Z").fillet(0.125)
```

`show_object()` is not used here so the fallback behavior is to render all object declared in variables. This can be a source of confusion for a beginner, who may think that there is no such function and the model to render must have to be declared as the `result` variable.

I suggest this (or similar):

```python
import cadquery as cq

box = cq.Workplane("XY" ).box(3, 3, 0.5)
show_object(box.edges("|Z").fillet(0.125))
```