BackdropScaffold gif example update!

[sachin-dahal]

An issue #95 was that the gif example was not matching to the example shown on the main documentation page by which confusion might occur to a few.
Here is the comparison between the two example gifs which is now updated with the relevant one for the mentioned example code on the main documentation page

BackdropScaffold(
  appBar: BackdropAppBar(
    title: Text("Backdrop Example"),
    actions: <Widget>[
      BackdropToggleButton(
        icon: AnimatedIcons.list_view,
      )
    ],
  ),
  backLayer: Center(
    child: Text("Back Layer"),
  ),
  frontLayer: Center(
    child: Text("Front Layer"),
  ),
)

Before

After

[WieFel]

LGTM
@daadu ?

[daadu]

This has been pushed to pub.dev with v0.6.2.

@sachin-dahal Thanks for the contribution, check yourself in README!!