pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/guess-the-number/activity.md

@@ -2,11 +2,11 @@
 
 Guess the number with math random. 
 
-### ~avatar avatar
+## ~avatar avatar
 
 Welcome! This tutorial will help you create a guess the number game! Let's get started!
 
-### ~
+## ~
 
 Add an event handler when button `A` is pressed.
 
@@ -37,9 +37,9 @@ input.onButtonPressed(Button.A, () => {
 
 ```
 
-### ~avatar avatar
+## ~avatar avatar
 
 Excellent, you're ready to continue with the [challenges](/lessons/guess-the-number/challenges)!
 
-### ~
+## ~
 

docs/lessons/guess-the-number/challenges.md

@@ -13,7 +13,7 @@ input.onButtonPressed(Button.A, () => {
 })
 ```
 
-### Challenge 1
+## Challenge 1
 
 When button `B` is pressed, we want to clear the screen. This will make it so users can play your game over and over again! Add an event handler to handle this case.
 
@@ -27,7 +27,7 @@ input.onButtonPressed(Button.B, () => {
 })
 ```
 
-### Challenge 2
+## Challenge 2
 
 Show an animation when you clear the screen! Choose what animation makes most sense to you. Be creative!
 

docs/lessons/guess-the-number/tutorial.md

@@ -1,12 +1,12 @@
 # guess the number tutorial
 
-### ~avatar avatar
+## ~avatar avatar
 
 This tutorial will help you create a guess the number game! Let's get started!
 
-### ~
+## ~
 
-### Rebuild the game!
+## Rebuild the game!
 
 The blocks have been shuffled! Put them back together so that...
 * when the user presses button ``A``,