pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/dice-roll/challenges.md

@@ -60,7 +60,7 @@ input.onGesture(Gesture.Shake, () => {
 });
 ```
 
-### Challenge 1
+## Challenge 1
 
 Modify the line of code with `pick random` so that only number 1-4 can appear on the dice.
 
@@ -119,7 +119,7 @@ input.onGesture(Gesture.Shake, () => {
 });
 ```
 
-### Challenge 2
+## Challenge 2
 
 Let's make a trick dice! Modify the line of code with `pick random` so that only numbers 3-6 can appear on the dice. Also note that we need to ensure `roll = 0` when only 1 dot is shown on the @boardname@.
 
@@ -177,7 +177,7 @@ input.onGesture(Gesture.Shake, () => {
 });
 ```
 
-### Challenge 3
+## Challenge 3
 
 Add a couple more conditions so that the @boardname@ randomly chooses a number between 1 and 8.