docs: Fix typo in flame_bloc readme

[abgutierrez]

the example should be

class ExampleGame extends FlameBlocGame {
  void selectWeapon() {
    read<InventoryBloc>.add(WeaponSelected('axe'));
  }
}

not

class ExampleGame example FlameBlocGame {
  void selectWeapon() {
    read<InventoryBloc>.add(WeaponSelected('axe'));
  }
}

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