rename micro:bit to @boardname@

docs/lessons/game-counter/challenges.md

@@ -33,5 +33,5 @@ input.onButtonPressed(Button.B, () => {
 
 ### Challenge 3
 
-Now let's try to reset the counter when the micro:bit is shaken. You will need to register an event handler with `on shake`.
+Now let's try to reset the counter when the @boardname@ is shaken. You will need to register an event handler with `on shake`.