Bug in Asteroids sample program

[microbit-giles]

This is an awesome project! I've just been using it with the multiple files in project feature of the new micro:bit python editor, https://python.microbit.org/ - and I discovered a little bug in the Asteroid game sample, line 18 refers to showing a bitmap called 888.bin, I think it should say 'splash' instead. show_bitmap("splash") not show_bitmap("888.bin")