Update simple-game-extended.md

[iain-davis]

There was a final step missing, where the new Drop(Game) class needed to be added to the launcher.

[Frosty-J]

I think there's some confusion because in the old version of "A Simple Drop" (prior to #228) the main class was Drop instead of Main, which never got changed for "Extending the Simple Game". The original intention appeared to be to just replace the existing game.