remove Builder widget

Thank you for creating this widget.

One question on that is about implementation.

Regarding the part written in doc installing 3, don't you need the Builder widget?


## Here is an example

### before
```dart
ShowCaseWidget(
  builder: Builder(
    builder : (context)=> Somewidget()
  ),
),
```

### after
```dart
ShowCaseWidget(
  builder : (context)=> Somewidget()
),
```

