Fix example in README: replace EdgeInsets with HtmlPaddings in Style.padding

[KagemaNjoroge]

The example in the README used EdgeInsets.all(16) for Style.padding, which causes a type error since the parameter expects an HtmlPaddings instance. This PR updates the example to use HtmlPaddings.all(16) instead. Also removes an unnecessary const from the first attempt.

@erickok @Sub6Resources