Usage with angular, printScreen does not work without view arg

Hello, it looks like printScreen() without any view arg doesn't work in angular based projects. `let view: View = frame.topmost().currentPage.content;` ends up being undefined. Passing the view explicitly works but has no example. This works:

```
@ViewChild("foo") foo: ElementRef;
printer.printScreen({ view: this.foo.nativeElement });
```

I could submit a PR just updating the documentation if you'd like. If we wanted to not fix the currentPage issue, we could just note that the view arg is required when using Angular.