pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/counter/activity.md

@@ -2,11 +2,11 @@
 
 Display a number with a variable. 
 
-### ~avatar avatar
+## ~avatar avatar
 
 Welcome! This tutorial will teach you how to make a counter that increments when button A is pressed. Let's get started!
 
-### ~
+## ~
 
 Let's start by creating a **local variable** `count` to keep track of the current count.
 
@@ -35,9 +35,9 @@ input.onButtonPressed(Button.A, () => {
 ```
 
 
-### ~avatar avatar
+## ~avatar avatar
 
 Excellent, you're ready to continue with the [challenges](/lessons/counter/challenges)
 
-### ~
+## ~
 

docs/lessons/counter/challenges.md

@@ -14,7 +14,7 @@ input.onButtonPressed(Button.A, () => {
 })
 ```
 
-### Challenge 1
+## Challenge 1
 
 Let's add the code to `count` when `B` is pressed. Add an event handler with `on button pressed(B)` then add the code to `count`.
 
@@ -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`.