pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/game-counter/activity.md

@@ -24,9 +24,9 @@ input.onButtonPressed(Button.A, () => {
 
 
 
-### ~avatar boothing
+## ~avatar boothing
 
 Excellent, you're ready to continue with the [challenges](/lessons/game-counter/challenges)!
 
-### ~
+## ~
 

docs/lessons/game-counter/challenges.md

@@ -13,7 +13,7 @@ input.onButtonPressed(Button.A, () => {
 });
 ```
 
-### Challenge 1
+## Challenge 1
 
 Let's add the code to `score` when `B` is pressed. Add an event handler with `on button (B) pressed` then add the code to `score`.
 
@@ -31,7 +31,7 @@ input.onButtonPressed(Button.B, () => {
 ```
 
 
-### Challenge 3
+## Challenge 3
 
 Now let's try to reset the counter when the @boardname@ is shaken. You will need to register an event handler with `on shake`.