Fix documentation on 'Using packages'

[YoungChulShin]

1. Fix example code indentation
2. Modify first example code content. (Content is duplicated)
   • Before : It is valid for second example

     Run the app. When you click the 'Show Flutter homepage' you should see the phone's default browser open, and the Flutter homepage appear.
     

   • After

     Run the app. You can see app's background color is orange.
     

[sfshaza2]

Thanks, @YoungChulShin, for catching this!! I've made a lot of other edits to this file, so it's fixed in PR #2711