build() instead of __init__() in TodoApp class examples

[jeasoft]

I got the next issue while following the tutorial: TypeError: __init__() should return None, not 'Column'. It seems the TodoApp() class have build()method instead of the __init__()

BTW, thanks guys for your awesome work, I'm exploring this framework to prototype a mobile app and it seems it's a great option for those who are not very familiar with Dart stuff

[CLAassistant]

All committers have signed the CLA.

[ndonkoHenri]

Thanks for your contribution! This section of the tutorial actually needs to be updated because UserControl is now deprecated.

[InesaFitsner]

updated tutorial to get rid of UserControl and this change was included